WASHINGTON/SEOUL (Reuters) - The United States said on Wednesday it would soon send a missile defense system to Guam to defend it from North Korea, as the U.S. military adjusts to what Defense Secretary Chuck Hagel has called a "real and clear danger" from Pyongyang. 
 
 At the same time, North Korea repeated its threat to launch a nuclear attack on the United States. Pyongyang said it had ratified a potential strike because of U.S. military deployments around the Korean peninsula that it claimed were a prelude to a possible nuclear attack on the North. 
 
 Washington had been informed of the potential attack by North Korea, a spokesman for its army said in a statement carried by the English language service of state news agency KCNA. It was unclear how such a warning was given since North Korea does not have diplomatic ties with Washington. 
 
 Experts say North Korea is years away from being able to hit the continental United States with a nuclear weapon, despite having worked for decades to achieve nuclear-arms capability. 
 
 North Korea has previously threatened a nuclear strike on the United States and missile attacks on its Pacific bases, including in Guam, a U.S. territory in the Pacific. Those threats followed new U.N. sanctions imposed on the North after it carried out its third nuclear test in February. 
 
 "Some of the actions they've taken over the last few weeks present a real and clear danger," Hagel told an audience at the National Defense University in Washington. 
 
 U.S. stocks sank to their lows of the day after Hagel's comments and the Guam deployment news. 
 
 The yield on the benchmark 10-year U.S. Treasury note briefly dropped below 1.8 percent for the first time since January as investors sought safety in government bonds, driving their prices higher. The dollar strengthened modestly against the euro and yen. 
 
 The South Korean won hit a six-month low on Wednesday. 
 
 "I would say that people are taking it a lot more seriously than they used to," said Steve Van Order, a fixed income strategist at Calvert Investments in Maryland, referring to the tensions with North Korea. 
 
 Despite the rhetoric, Pyongyang has not taken any military action and has shown no sign of preparing its 1.2 million strong armed forces for war, the White House said on Monday. 
 
 That would indicate that its threats are partly intended for domestic consumption to bolster young leader Kim Jong-un ahead of celebrations marking the anniversary of the birthday of Kim Il-sung, the state's founder and the younger Kim's grandfather, on April 15. 
 
 Caitlin Hayden, spokeswoman for the White House National Security Council, criticized the latest North Korean statement. 
 
 "It is yet another offering in a long line of provocative statements that only serve to further isolate North Korea from the rest of the international community and undermine its goal of economic development," Hayden said. 
 
 North Korea had "ratified" a merciless attack against the United States, potentially involving a "diversified nuclear strike", the KCNA statement concluded after railing against annual war games between South Korea and the United States that run throughout April. 
 
 "The moment of explosion is approaching fast. No one can say a war will break out in Korea or not and whether it will break out today or tomorrow," the KCNA statement said. 
 
 HAGEL: MUST TAKE THREATS SERIOUSLY 
 
 Hagel said he had to take the threats seriously, language he has used in recent weeks as the United States has revamped its missile defense plans and positioned two guided-missile destroyers in the western Pacific to bolster missile defense. 
 
 The United States has also flexed its muscle during the military drills with South Korea, flying two radar-evading stealth bombers on a first-of-its-kind practice bombing run over South Korea. 
 
 In the latest move, the Pentagon said it was deploying a Terminal High Altitude Area Defense system to Guam in the coming weeks. The THAAD system includes a truck-mounted launcher, interceptor missiles and an AN/TPY-2 tracking radar. 
 
 Last month, Hagel said the Pentagon would add 14 new anti-missile interceptors in Alaska and move ahead with the deployment of a second missile-defense radar in Japan. 
 
 Adding to tensions, North Korea on Wednesday closed access to a joint factory zone with South Korea, putting at risk $2 billion a year in trade that is vital to the impoverished state. 
 
 It said on Tuesday it would revive a mothballed nuclear reactor able to produce bomb-grade plutonium. 
 
 Hagel called America's responses so far "measured, responsible, serious" and also said the United States was working with allies to lower tensions. 
 
 "We are doing everything we can, working with the Chinese, others to defuse that situation on the peninsula," he said. 
 
 In Beijing, China's deputy foreign minister met ambassadors from the United States and both Koreas to express "serious concern" about the Korean peninsula, China's Foreign Ministry said on Wednesday. 
 
 It was a sign that China, the North's major benefactor, was increasingly worried about events spinning out of control. 
 
 (Additional reporting by David Alexander in Washington and Caroline Valetkevitch in New York. Writing by Dean Yates) ||||| SEOUL, South Korea — The United States announced Wednesday that it was speeding the deployment of an advanced missile defense system to Guam in the next few weeks, two years ahead of schedule in what the Pentagon said was “a precautionary move” to protect American naval and air forces from the threat of a North Korean missile attack. 
 
 The system — called Thaad, for Terminal High Altitude Area Defense — was scheduled for deployment around 2015. 
 
 The decision to deploy it now was the latest in a series of steps intended to deter the North from either military action or new missile tests and came only hours after the latest North Korean provocation, with officials blocking South Koreans from crossing the border to enter a jointly operated industrial park. 
 
 The North had threatened the move in reaction to taunts from the South Korean news media that the North Korean leader, Kim Jong-un, had cut hot lines and other communications across the border, but did not want to risk one of his most precious sources of hard currency. The border has been sealed before, but the move against the operations at the industrial park that employs roughly 53,000 North Koreans raised doubt about the future of the last remaining major symbol of inter-Korean cooperation. 
 
 In recent weeks, the North has repeatedly threatened that, if provoked, it could target United States forces in Guam and Hawaii as well as the mainland United States — a threat it repeated Thursday. 
 
 Earlier this week, the Defense Department announced that two of the Navy’s Aegis-equipped missile defense warships were positioned in the Pacific to watch North Korea. Installing the land-based missile system in Guam will free up the ships, which have radar and interceptor missiles, to be repositioned closer to the North Korean coast. 
 
 That would give President Obama a wider range of options if the North Koreans fire their missiles in a test or at a target. 
 
 “We haven’t made any decisions,” a senior administration official said. “But we want as many options as possible.” 
 
 The last time the United States seriously prepared to shoot down North Korean missiles was the summer of 2006, when the defense secretary at the time, Donald H. Rumsfeld, ordered the Army to prepare to intercept a long-range Taepodong missile from its antiballistic missile base in Alaska during a North Korean test. But the North Korean missile broke up in flight. 
 
 Last month, as the North escalated its threats, Defense Secretary Chuck Hagel announced that the United States would bolster long-range ballistic missile defenses in Alaska and California. But that process will take several years; the Thaad is intended to deter a threat to Guam, which is considered to be on the outer edge of the North’s missile range. The system includes a truck-mounted launcher, interceptor missiles, an integrated fire control system and advanced tracking radar. 
 
 Mr. Hagel, speaking at the National Defense University in Washington, referred to North Korea’s increased nuclear ability in response to a question from the audience. 
 
 “They have a nuclear capacity now,” he said. “They have a missile delivery capacity now. And so, as they have ratcheted up their bellicose, dangerous rhetoric, and some of the actions they have taken over the last few weeks present a real and clear danger.” 
 
 Mr. Hagel’s carefully worded comment about the North’s “nuclear capacity” was significant; on Tuesday, Secretary of State John Kerry insisted that the United States would never recognize the North “as a nuclear state.” The difference pointed to the administration’s dilemma: after three nuclear tests, there is no doubt the country can trigger a nuclear explosion, but the United States is adamant that it will not reward the North by accepting its arsenal as a permanent reality. 
 
 In recent days the North has said it would never negotiate away its nuclear weapons arsenal, and it has taken steps toward expanding it. It declared it would restart a nuclear reactor that gave it a small stockpile of plutonium. Photographs published Wednesday on the Web site 38 North, which follows North Korean developments, show new construction at the aging reactor, dating back several weeks. 
 
 The jointly run industrial complex, in the North Korean town of Kaesong, had continued to operate for days since the North threatened to shut it down. But on Wednesday, more than 480 South Koreans who showed up at a border crossing were denied permission to cross, said the Unification Ministry of South Korea, which is in charge of relations with the North. North Korea promised to allow 861 South Koreans currently staying in Kaesong to return home if they wished, the ministry said. But with no replacements arriving, only 33 immediately decided to return home. 
 
 Choe Sang-hun reported from Seoul, and David E. Sanger from Washington. Mark Landler and Thom Shanker contributed reporting from Washington, and Patrick Zuo from Beijing. ||||| North Korea on Wednesday barred South Korean workers from entering a jointly run factory park just over the heavily armed border in the North, officials in Seoul said, a day after Pyongyang announced it would restart its long-shuttered plutonium reactor and increase production of nuclear weapons material. 
 
 South Korean army soldiers walk on the empty road after South Korean vehicles which were refused for entry to North Korea at the customs, immigration and quarantine office in Paju, South Korea, near the... (Associated Press) 
 
 South Korean vehicles turn back their way as they were refused for entry to North Korea's city of Kaesong, at the customs, immigration and quarantine office in Paju, South Korea, near the border village... (Associated Press) 
 
 South Korean Marine K-55 self-propelled howitzers are on positions during an exercise against possible attacks by North Korea near the border village of Panmunjom in Paju, South Korea, Wednesday, April... (Associated Press) 
 
 South Korean vehicles turn back their way as they were refused for entry to North Korea's city of Kaesong, at the customs, immigration and quarantine office in Paju, South Korea, near the border village... (Associated Press) 
 
 South Korean Marines pass by K-55 self-propelled howitzers during an exercise against possible attacks by North Korea near the border village of Panmunjom in Paju, South Korea, Wednesday, April 3, 2013.... (Associated Press) 
 
 The move to block South Koreans from going to their jobs at the Kaesong industrial complex, the last remaining symbol of detente between the rivals, comes amid increasing hostility from Pyongyang, which has threatened to stage nuclear and missile strikes on Seoul and Washington and has said that the armistice ending the 1950s Korean War is void. 
 
 Seoul's Unification Ministry spokesman Kim Hyung-suk said Pyongyang was allowing South Koreans to return home from Kaesong. Three workers returned Wednesday morning; dozens more were scheduled to return later. But Kim said about 480 South Koreans who had planned to travel to the park Wednesday were being refused entry. 
 
 North Korean authorities cited recent political circumstances on the Korean Peninsula when they delivered their decision to block South Korean workers from entering Kaesong, Kim said without elaborating. 
 
 It's the latest sign of deepening tensions on the Korean Peninsula. North Korea said Tuesday that it will quickly begin "readjusting and restarting" the facilities at its main Nyongbyon nuclear complex, including the plutonium reactor and a uranium enrichment plant. Both could produce fuel for nuclear weapons. Analysts saw the statement as Pyongyang's latest attempt to extract U.S. concessions by raising fears of war. Experts estimate reactivating the reactor could take anywhere from three months to a year. 
 
 The rising tide of threats in recent weeks are seen as efforts by the North to force new policies in Seoul, diplomatic talks with Washington and to increase domestic loyalty to young North Korean leader Kim Jong Un by portraying him as a powerful military commander. 
 
 North Korea is angry about ongoing South Korea-U.S. military drills and new U.N. sanctions over its Feb. 12 nuclear test, its third. The Korean Peninsula technically remains in a state of war because a truce, not a peace treaty, ended the Korean War. The United States stations 28,500 troops in South Korea as a deterrent to North Korea. 
 
 The North's plutonium reactor began operations in 1986 but was shut down as part of international nuclear disarmament talks in 2007 that have since stalled. Tuesday's nuclear announcement underscores worries about North Korea's timetable for building a nuclear-tipped missile that can reach the United States, although it is still believed to be years away from developing that technology. 
 
 The North's rising rhetoric has been met by a display of U.S. military strength, including flights of nuclear-capable bombers and stealth jets at the annual South Korean-U.S. military drills that the allies call routine but that North Korea claims are invasion preparations. 
 
 The Kaesong industrial park started producing goods in 2004 and has been an unusual point of cooperation in an otherwise hostile relationship between the Koreas. 
 
 North and South Korea do not allow their citizens to travel to the other country without approval, but an exception had previously been made each day for the South Koreans working at Kaesong. 
 
 About 120 South Korean firms run factories in the border town of Kaesong, with 53,000 North Koreans working there. Using North Korea's cheap, efficient labor, the Kaesong complex produced $470 million worth of goods last year. 
 
 Pyongyang threatened last week to shut down the park, which is run with mostly North Korean labor and South Korean know-how. It expressed anger over South Korean media reports that said North Korea hadn't yet shut the park because it is a source of crucial hard currency for the impoverished country. 
 
 In 2009, North Korea closed its border gate in anger over U.S.-South Korean military drills, leaving hundreds of South Korean workers stranded in Kaesong for several days. The park later resumed normal operations. 
 
 "I feel worried that I'm unable to do business and also feel anxious," Joe In-suk, a 54-year-old South Korean who had planned to travel to Kaesong on Wednesday, said at a border checkpoint in Paju, South Korea. About a dozen South Korean trucks were lined up at the checkpoint leading into North Korea. 
 
 If North Korea continues to deny entrance to South Korean workers, it could be tantamount to a shutdown because Kaesong factories cannot operate production lines without supplies of raw materials sent regularly by truck from the South to the North. 
 
 A South Korean manager whose company runs a factory in Kaesong was worried that buyers would drop future orders if North Korea continued to block workers and supplies from the South. 
 
 "For some companies, today's move must have already dealt them a blow," the manager said Wednesday, speaking on condition of anonymity because he wasn't authorized to talk to the media. "We cannot produce goods without raw material." 
 
 Seoul's Unification Ministry urged Pyongyang to "immediately normalize" cross-border traffic in and out of Kaesong. 
 
 The U.S., meanwhile, called for North Korea to abandon its nuclear ambitions, saying it would be "extremely alarming" if Pyongyang follows through on a vow to restart its plutonium reactor. 
 
 White House spokesman Jay Carney said the U.S. is taking steps to ensure it has the capacity to defend itself and its allies. 
 
 But Carney noted that a string of threats from North Korea toward the U.S. and South Korea so far have not been backed up by action, calling the threats part of a counterproductive pattern. He called on Russia and China, two countries he said have influence on North Korea, to use that influence to persuade the North to change course. 
 
 U.S. Defense Secretary Chuck Hagel called North Korea's development of nuclear weapons a growing threat. In a telephone call Tuesday evening to Chinese Defense Minister Chang Wanquan, Hagel said Washington and Beijing should continue to cooperate on North Korea's pursuit of nuclear weapons and ballistic missiles. 
 
 China, North Korea's only major economic and diplomatic supporter, expressed unusual disappointment with its ally. 
 
 Hwang Jihwan, a North Korea expert at the University of Seoul, said the North "is keeping tension and crisis alive to raise stakes ahead of possible future talks with the United States." 
 
 "North Korea is asking the world, `What are you going to do about this?'" he said. 
 
 The North's nuclear statement Tuesday suggests it will do more to produce highly enriched uranium. The technology needed to make highly enriched uranium bombs is much easier to hide than huge plutonium facilities. North Korea previously insisted that its uranium enrichment was for producing electricity _ meaning low-enriched uranium. 
 
 Kim Jin Moo, a North Korea expert at the Korea Institute for Defense Analyses in South Korea, said that by announcing it is "readjusting" all nuclear facilities, including the uranium enrichment plant, North Korea "is blackmailing the international community by suggesting that it will now produce weapons-grade, highly enriched uranium." 
 
 The North's plutonium reactor was disabled under a 2007 deal made at now-dormant aid-for-disarmament negotiations involving the North, the U.S., South Korea, Japan, China and Russia. 
 
 In 2008, North Korea destroyed the cooling tower at Nyongbyon in a show of commitment, but the deal later stalled after the North balked at allowing intensive international fact-checking of its past nuclear activities. North Korea pulled out of the talks after condemnation of its long-range rocket launch in April 2009. 
 
 North Korea is believed to have exploded plutonium devices in its first two nuclear tests, in 2006 and 2009. 
 
 There had long been claims by the U.S. and others that North Korea was also pursuing a secret uranium program. In 2010, the North unveiled to visiting Americans a uranium enrichment program at Nyongbyon. 
 
 Analysts say they don't believe North Korea currently has mastered the miniaturization technology needed to build a warhead that can be mounted on a missile, and the extent of its uranium enrichment efforts is also unclear. Some experts estimate North Korea may have enough plutonium for perhaps four to eight rudimentary bombs. 
 
 ___ 
 
 Associated Press writers Foster Klug, Hyung-jin Kim and Youkyung Lee in Seoul and Kim Yong-ho in Paju contributed to this report. 
 
 ___ 
 
 Follow Foster Klug on Twitter at twitter.com/APKlug; Sam Kim at twitter.com/samkim_ap; Youkyung Lee at twitter.com/YKLeeAP
– Some action from North Korea after weeks of harsh words: Pyongyang has blocked nearly 500 South Korean workers from entering a jointly run factory park just over the heavily armed border in the North today. As Reuters reports, the move to block South Koreans from going to their jobs at the Kaesong Industrial Park, dubbed the "last remaining symbol of detente" between the rivals by the AP, puts a vital source of revenue for the North at risk. As such, some South Korean experts believe the move could be a short-lived one. And signs suggest the factories haven't been shuttered: Though the North has given the OK for the 861 South Koreans working in the zone to travel home, as of today fewer than 40 had done so, reports the New York Times. However, Reuters notes that the South transports all supplies to the complex, so food stocks could begin to dwindle. Entry to the park, which is home to 123 factories, was previously blocked three times in 2009, once for a three-day period. The move comes amid increasing hostility from Pyongyang, include yesterday's threat to restart its sole nuclear reactor.