The problem has always been what China will bear in terms of restricting its protégé and neighbor, as well as whether it will cut back fuel shipments and other trade with North Korea. 
 
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 “Moving forward, China really holds the key to what extent the actions will be different this time,” said Suzanne DiMaggio, an expert at the Asia Society. 
 
 The signs are hard to read. 
 
 China will almost certainly join the United States in supporting tougher sanctions over Tuesday’s test, accompanied by sterner reprimands from Beijing against its recalcitrant ally in Pyongyang, which ignored Chinese entreaties not to take provocative actions. 
 
 But as impatient as China might be with North Korea, there is little chance that the new Chinese leader, Xi Jinping, will move quickly to change the nation’s long-held policy of propping up the walled-off government that has long served as a buffer against closer intrusion by the United States on the Korean Peninsula. 
 
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 The Chinese military, and to a lesser extent the International Liaison Department of the Chinese Communist Party, assert strong influence on China’s Korea policy, and both powerful entities prefer to keep North Korea close at hand, Chinese and American analysts say. 
 
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 While the People’s Liberation Army is not even able to conduct military exercises with the North Koreans — the government in the North forbids such contact with outsiders — Chinese military strategists adhere to the doctrine that they cannot afford to abandon their ally, no matter how bad its behavior, analysts here say. 
 
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 At the same time, the Chinese Communist Party looks upon the North Korean Communist Party — led by Kim Jong-un, the grandson of the nation’s founder — as a fraternal brotherhood. Indeed, relations between the two countries are conducted largely between the two parties rather than between the two foreign ministries, the more normal diplomatic channel. 
 
 In an early sign that Mr. Xi is unlikely to veer from past policy, the state-run news agency, Xinhua, criticized the United States and its allies for essentially forcing the North’s aggression by causing the country to feel insecure. 
 
 But within this basic contour there could be some adjustments by Mr. Xi, according to Zhu Feng, a professor of international relations at Peking University, an advocate of a tougher policy by China against North Korea. 
 
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 “One nuclear test will not make China’s new administration decide to ‘abandon North Korea,’ but it will definitely worsen China-North Korea relations,” Professor Zhu wrote in a recent article in The Straits Times of Singapore. “North Korea’s nuclear test will make the new Xi Jinping administration angry, and give China a headache.” 
 
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 Mr. Xi, who became head of the Communist Party and military council in November, will ascend to the presidency of the country next month. 
 
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 To improve China’s strained relationship with the United States, Mr. Xi — who has shown himself to be more nationalistic than his predecessor — could start with getting tougher on North Korea, including at the Security Council. 
 
 The Obama administration excoriated Mr. Xi’s predecessor, Hu Jintao, after North Korea’s second nuclear test in 2009, accusing him of “willful blindness” to that country’s actions. 
 
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 “With Hu out of the picture, the administration is intent on determining whether Xi Jinping will prove more attentive to U.S. security concerns,” said Jonathan D. Pollack, director of the John L. Thornton China Center at the Brookings Institution. 
 
 “How Xi chooses to respond will be an important early signal of his foreign policy priorities and whether he is ready to cooperate much more openly and fully with Washington and Seoul than his predecessor,” he said, referring to South Korea. 
 
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 China’s calculations will be crucial to what happens at the Security Council, where the policy has always been to pursue unanimity over toughness; it is considered far better to get all members on board to send a message to North Korea rather than have China abstain or worse, veto. 
 
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 In the absence of any real leverage, Washington and its allies are left warning Beijing that if it does not keep North Korea out of the nuclear club, it risks an arms race in its own neighborhood. 
 
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 President Obama spoke Tuesday morning, hours after the test, with South Korea’s outgoing president, Lee Myung-bak, in what the White House described as an effort to agree on a common strategy at the Security Council and in the two country’s military posture on the Korean Peninsula, where troops have faced off against the North since the armistice was signed 60 years ago. 
 
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 A statement issued by the White House after the conversation included a rare reference to American’s “nuclear umbrella” over South Korea, a reminder to the North that an attack on South Korea would be viewed as an attack on the United States. While that has been true for decades, it is rarely highlighted in American statements. 
 
 “Threatening a missile-capable warhead with a successful third nuclear test gives the United States, South Korea and Japan good reason to step up their regional ballistic missile defense capabilities,” said Siegfried S. Hecker, former director of the Los Alamos National Laboratory and co-director of the Center for International Security and Cooperation at Stanford University. “That should give the Chinese government some pause.” 
 
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 The United States faces its own geopolitical calculations. Some experts say it needs to keep up the tough talk, even if it understands that its efforts at the Security Council may not do much to limit the North’s capabilities. Even the strongest sanctions and increasing isolation have not caused the North to back off its nuclear program, which the leadership sees as both a deterrent to possible American aggression, and which gives the impoverished country’s governing elite a success it can show its suffering people. 
 
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 Now experts say the North may be simply trying to wait the United States out, hoping it will eventually recognize its program as it did Pakistan’s. But experts say Mr. Obama cannot recognize North Korea as the nuclear state it increasingly is without giving signals to Iran — which the West believes is pursuing a nuclear weapon despite its denials — that it could try the same path. 
 
 As the world’s powers struggle to refine their policies, North Korea continues to make technological advances. A long-range rocket test in December has been judged by outside experts to have been a success after many failures. 
 
 And Tuesday’s test, if it was as large as originally thought, appears to suggest that the nuclear arms program is also moving ahead. (The first test in 2006 was considered a partial failure.) 
 
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 “It moves the question of North Korea as a nuclear contender from ‘if’ to ‘when,’ ” said one senior Obama administration official. “The ‘when’ may still be years away, but at least now it is in sight.” ||||| SEOUL North Korea conducted its third nuclear test on Tuesday in defiance of U.N. resolutions, drawing condemnation from around the world, including from its only major ally, China, which summoned the North Korean ambassador to protest. 
 
 Pyongyang said the test was an act of self-defense against "U.S. hostility" and threatened stronger steps if necessary. 
 
 The test puts pressure on U.S. President Barack Obama on the day of his State of the Union speech and also puts China in a tight spot, since it comes in defiance of Beijing's admonishments to North Korea to avoid escalating tensions. 
 
 The U.N. Security Council held an emergency meeting at which its members, including China, "strongly condemned" the test and vowed to start work on appropriate measures in response, the president of the council said. 
 
 North Korean leader Kim Jong-un, the third of his line to rule the country, has presided over two long-range rocket launches and a nuclear test during his first year in power, pursuing policies that have propelled his impoverished and malnourished country closer to becoming a nuclear weapons power. 
 
 North Korea said the test had "greater explosive force" than those it conducted in 2006 and 2009. Its KCNA news agency said it had used a "miniaturized" and lighter nuclear device, indicating it had again used plutonium, which is suitable for use as a missile warhead. 
 
 China, which has shown signs of increasing exasperation with the recent bellicose tone of its reclusive neighbor, summoned the North Korean ambassador in Beijing and protested sternly, the Foreign Ministry said. 
 
 Foreign Minister Yang Jiechi said China was "strongly dissatisfied and resolutely opposed" to the test and urged North Korea to "stop any rhetoric or acts that could worsen situations and return to the right course of dialogue and consultation as soon as possible". 
 
 Analysts said the test was a major embarrassment to China, which is a permanent member of the Security Council and North Korea's sole major economic and diplomatic ally. 
 
 Obama called the test a "highly provocative act" that hurt regional stability. 
 
 "The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community. The United States will also continue to take steps necessary to defend ourselves and our allies," Obama said. 
 
 U.S. Ambassador to the United Nations Susan Rice said Washington and its allies intended to "augment the sanctions regime" already in place due to Pyongyang's previous atomic tests. North Korea is already one of the most heavily sanctioned states in the world and has few external economic links that can be targeted. 
 
 Japanese Prime Minister Shinzo Abe said the test was a "grave threat" that could not be tolerated. 
 
 Russian Foreign Minister Sergei Lavrov urged North Korea to abandon its nuclear arms program and return to talks. NATO condemned the test as an "irresponsible act." 
 
 South Korea, still technically at war with North Korea after a 1950-53 civil war ended in a mere truce, also denounced the test. Obama spoke to South Korean President Lee Myung-bak on Tuesday and told him the United States "remains steadfast in its defense commitments" to Korea, the White House said. 
 
 MAXIMUM RESTRAINT 
 
 North Korea's Foreign Ministry said the test was "only the first response we took with maximum restraint". 
 
 "If the United States continues to come out with hostility and complicates the situation, we will be forced to take stronger, second and third responses in consecutive steps," it said in a statement carried by the official KCNA news agency. 
 
 North Korea - which gave the U.S. State Department advance warning of the test - often threatens the United States and its "puppet", South Korea, with destruction in colorful terms. 
 
 North Korea told the U.N. disarmament forum in Geneva that it would never bow to resolutions on its nuclear program and that prospects were "gloomy" for the denuclearization of the divided Korean peninsula because of a "hostile" U.S. policy. 
 
 Suzanne DiMaggio, an analyst at the Asia Society in New York, said North Korea had embarrassed China with the test. "China's inability to dissuade North Korea from carrying through with this third nuclear test reveals Beijing's limited influence over Pyongyang's actions in unusually stark terms," she said. 
 
 Mark Fitzpatrick of the International Institute for Strategic Studies think tank, said: "The test is hugely insulting to China, which now can be expected to follow through with threats to impose sanctions." 
 
 The magnitude of the explosion was roughly twice that of the 2009 test, according to the Vienna-based Comprehensive Nuclear-Test Ban Treaty Organization. The U.S. Geological Survey said that a seismic event measuring 5.1 magnitude had occurred. 
 
 U.S. intelligence agencies were analyzing the event and found that North Korea probably conducted an underground nuclear explosion with a yield of "approximately several kilotons", the Office of the Director of National Intelligence said. 
 
 Nuclear experts have described Pyongyang's previous two tests as puny by international standards. The yield of the 2006 test has been estimated at less than 1 kiloton (1,000 tons of TNT equivalent) and the second at some 2-7 kilotons, compared with 20 kilotons for a Nagasaki-type bomb. 
 
 Initial indications are that the test involved the latest version of a plutonium-based prototype weapon, according to one current and one former U.S. national security official. Both previous tests involved plutonium. If it turns out the test was of a new uranium-based weapon, it would show that North Korea has made more progress on uranium enrichment than previously thought. 
 
 The United States uses WC-135 Constant Phoenix "sniffer" aircraft to collect samples to identify nuclear explosions. These would need to be deployed quickly to detect whether highly enriched uranium rather than plutonium was used because uranium decays to undetectable levels within a matter of days. Plutonium takes much longer to decay. 
 
 North Korea trumpeted news of the test on its state television channel to patriotic music against a backdrop of its national flag. 
 
 "It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment," KCNA said. 
 
 North Korea linked the test to its technical prowess in launching a long-range rocket in December, a move that triggered the U.N. sanctions, backed by China, that Pyongyang said prompted it to take Tuesday's action. 
 
 The North's ultimate aim, Washington believes, is to design an intercontinental ballistic missile capable of carrying a nuclear warhead that could hit the United States. North Korea says the program is aimed at putting satellites in space. 
 
 Despite its three nuclear tests and long-range rocket tests, North Korea is not believed to be close to manufacturing a nuclear missile capable of hitting the United States. 
 
 It used plutonium in previous nuclear tests and before Tuesday there had been speculation that it would use highly enriched uranium so as to conserve its plutonium stocks, as testing eats into its limited supply of materials to construct a nuclear bomb. 
 
 "VICIOUS CYCLE" 
 
 When Kim Jong-un, who is 30, took power after his father's death in December 2011, there were hopes that he would bring reforms and end Kim Jong-il's "military first" policies. 
 
 Instead, North Korea, whose economy is smaller than it was 20 years ago and where a third of children are believed to be malnourished, appears to be trapped in a cycle of sanctions followed by further provocations. 
 
 "The more North Korea shoots missiles, launches satellites or conducts nuclear tests, the more the U.N. Security Council will impose new and more severe sanctions," said Shen Dingli, a professor at Shanghai's Fudan University. "It is an endless, vicious cycle." 
 
 Options for the international community appear to be in short supply. Diplomats at the United Nations said negotiations on new sanctions could take weeks since China is likely to resist tough new measures for fear they could lead to further retaliation by the North Korean leadership. 
 
 Beijing has also been concerned that tougher sanctions could further weaken North Korea's economy and prompt a flood of refugees into China. 
 
 Tuesday's action appeared to have been timed for the run-up to February 16 anniversary celebrations of Kim Jong-il's birthday, as well as to achieve maximum international attention. 
 
 Significantly, the test comes at a time of political transition in China, Japan and South Korea, and as Obama begins his second term. The U.S. president will likely have to tweak his State of the Union address due to be given on Tuesday. 
 
 Japanese Prime Minister Shinzo Abe is bedding down a new government and South Korea's new president, Park Geun-hye, is preparing to take office on February 25. 
 
 China too is in the midst of a once-in-a-decade leadership transition to Xi Jinping, who takes office in March. Both Abe and Xi are staunch nationalists. 
 
 The longer-term game plan from Pyongyang may be to restart international talks aimed at winning food and financial aid. China urged it to return to the stalled "six-party" talks on its nuclear program, hosted by China and including the two Koreas, the United States, Japan and Russia. 
 
 Its puny economy and small diplomatic reach mean that North Korea struggles to win attention on the global stage - other than through nuclear tests and attacks on South Korea, the last of which was made in 2010. 
 
 "Now the next step for North Korea will be to offer talks... - any form to start up discussion again to bring things to their advantage," predicted Jeung Young-tae, senior research fellow at the Korea Institute for National Unification in Seoul. 
 
 (Additional reporting by Jack Kim, Christine Kim and Jumin Park in SEOUL; Linda Sieg in TOKYO; Louis Charbonneau and Michelle Nichols at the UNITED NATIONS; Fredrik Dahl in VIENNA; Michael Martina and Chen Aizhu in BEIJING; Mette Fraende in COPENHAGEN; Adrian Croft, Charlie Dunmore and Justyna Pawlak in BRUSSELS; Mark Hosenball, Paul Eckert, Roberta Rampton, Tabassum Zakaria and Jeff Mason in WASHINGTON; Editing by Nick Macfie, Claudia Parsons and David Brunnstrom) ||||| The statement from the Foreign Ministry reflects Beijing's growing frustration with its communist neighbor's provocative behavior, as well as its reluctance to impose more severe measures that could destabilize the North's hardline regime. 
 
 "The Democratic People's Republic of Korea, heedless of widespread international opposition, has again carried out a nuclear test, to which the Chinese government expresses its firm opposition," the statement said, referring to North Korea by its official name. 
 
 The statement said China maintained its resolute stance of advocating a nuclear-free Korean Peninsula, opposition to nuclear proliferation, and support for the maintenance of stability on the peninsula. It called on North Korea to abide by its denuclearization pledge, and not to "take additional actions that could cause the situation to further deteriorate." 
 
 The statement did not point to any specific actions Beijing would take in response to the nuclear test, the North's third. But it called on all sides to "respond calmly" and said issues should be resolved within the framework of long-stalled denuclearization talks involving North Korea, China, the U.S., South Korea, Japan and Russia. 
 
 China is the North's biggest source of aid and diplomatic support, but it issued an unusually strong reaction to Pyongyang's December rocket launch by agreeing to tightened United Nations sanctions on the country.
– In open defiance of the UN, North Korea has executed its third nuclear test, prompting global condemnation among leaders from the US to Russia to Japan. The latest test involved a "miniaturized and lighter nuclear device with greater explosive force than previously" in 2006 and 2009, the country said. Calling the test a "highly provocative act," President Obama has urged "swift and credible action by the international community," and the UN Security Council is planning a meeting this morning to address the issue, the New York Times reports. Pyongyang said afterward that the test was a "first response" to US hostility and warned of unspecified "second and third measures of greater intensity." The test has also angered China, Pyongyang's one major ally, Reuters notes. It's "hugely insulting to China, which now can be expected to follow through with threats to impose sanctions," says an analyst. Thus far, China has cited "firm opposition" to a test conducted "heedless of widespread international opposition," the AP reports. According to South Korea, seismic activity suggests the explosion measured some 6 to 7 kilotons—more than previous tests but less than the 20 kilotons detonated in Hiroshima. Seismic tests by several countries suggested the test, which the North called "safe and perfect," occurred near the location of its predecessors; the US Geological Survey says it was just a kilometer underground. Over at the Washington Post, the crack staff has found the exact test site—on Google Maps, just off "Nuclear Test Rd."