Nimitz-class aircraft carrier USS Ronald Reagan arrives at the Republic of Korea fleet base in Busan for a routine port visit as part of a regularly scheduled deployment. 
 
 SEOUL, South Korea — North Korea tried but failed to fire an intermediate-range missile, U.S. and South Korean officials said Sunday. 
 
 It was the first attempted missile launch since the North conducted its fifth and most powerful nuclear test Sept. 9, prompting a wave of international condemnation and efforts to impose new sanctions against the isolated country. 
 
 The missile test also occurred hours before the nuclear-powered USS Ronald Reagan aircraft carrier docked in the southeastern port city of Busan on Sunday for what the Navy called a routine visit after a week of maritime war games with the South Korean navy. 
 
 North Korea watchers have predicted in recent weeks that Pyongyang was preparing for another provocation, with satellite images showing activity at its main nuclear testing facility as well as its Sohae satellite-launching station. 
 
 The Pentagon said U.S. Strategic Command systems detected the failed launch of a presumed Musudan missile near the northwestern city of Kusong on Saturday. 
 
 Pentagon spokesman Gary Ross strongly condemned the missile test and stressed the U.S. commitment to defending allies South Korea and Japan is “ironclad.” 
 
 “This provocation only serves to increase the international community’s resolve to counter the DPRK’s prohibited activities,” he said, using the acronym for the North’s official name, the Democratic People’s Republic of Korea. 
 
 “We call on North Korea to refrain from actions that further raise tensions in the region,” he added. 
 
 South Korea’s military said the missile failed immediately after launch, according to the Yonhap news agency. The Joint Chiefs of Staff condemned what it called an “illegal provocation” that violated U.N. Security Council resolutions prohibiting the North from using ballistic missile technology. 
 
 North Korea has carried out an unprecedented number of missile tests this year, and experts say it is showing clear technical progress in its nuclear weapons program despite numerous failures. 
 
 Only one of the Musudan missiles, which have a range that puts them in reach of U.S. bases in Japan and Guam, is believed to have flown far enough to be considered a success. But experts say the North’s scientists learn from each failure. 
 
 Pyongyang has conducted five underground nuclear tests since 2006 , including two this year — one in January and the other on Sept. 9 — causing tensions to rise on the divided peninsula. 
 
 The U.S. and South Korea regularly conduct joint military exercises, which they insist are defensive in nature. North Korea, however, considers them a rehearsal for an invasion. It denounced the participation of the USS Ronald Reagan — the Navy’s only forward-deployed carrier — in naval exercises this week. 
 
 The Navy flew a group of reporters out to the ship 150 to 175 miles southwest of Osan Air Base on Friday to show off the carrier’s striking power. Nine fighter jets took off and 15 made landings during a 30-minute demonstration of FA-18 Super Hornets, E-2C Hawkeyes and the EA-18G Growler. 
 
 U.S. officials said the Ronald Reagan’s arrival in Busan was a routine port call that would allow the 5,500 crew members to engage with their South Korean counterparts. But Navy commanders stressed they were ready to defend the peninsula. 
 
 “We remain dedicated to the security of this nation … and this region,” Rear Adm. Brad Cooper, the commander of Naval Forces Korea, said in remarks after the ship docked. 
 
 He said the carrier strike group was there “to send a clear signal of the strength of our alliance and our resolve to protect (South Korea) from unprovoked acts of aggression.” 
 
 gamel.kim@stripes.com 
 
 Twitter: @kimgamel ||||| Image copyright Science Photo Library Image caption Negotiations over North Korea's nuclear programme, including the Yongbyon nuclear plant shown, have been a stop-start process 
 
 North Korea's nuclear programme remains a source of deep concern for the international community. Despite multiple efforts to curtail it, Pyongyang says it has conducted five nuclear tests. 
 
 Has North Korea got the bomb? 
 
 Image copyright AFP Image caption Pyongyang used a three-stage rocket to put a satellite into space in 2012 
 
 Technically yes - North Korea has conducted several tests with nuclear bombs. 
 
 However, in order to launch a nuclear attack on its neighbours, it needs to be able to make a nuclear warhead small enough to fit on to a missile. 
 
 While North Korea claims it has successfully "miniaturised" nuclear warheads, international experts have long cast doubt on these claims. 
 
 Yet according to information leaked to the Washington Post in August 2017, US intelligence officials now do believe North Korea is capable of miniaturisation. 
 
 How powerful are North Korea's nuclear bombs? 
 
 North Korea says it has conducted five successful nuclear tests: in 2006, 2009, 2013 and in January and September 2016. 
 
 The yield of the bombs appears to have increased. 
 
 September 2016's test has indicated a device with an explosive yield of between 10 and 30 kilotonnes - which, if confirmed, would make it the North's strongest nuclear test ever. 
 
 The other big question is whether the devices being tested are atomic bombs, or hydrogen bombs, which are more powerful. 
 
 H-bombs use fusion - the merging of atoms - to unleash massive amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms. 
 
 The 2006, 2009 and 2013 tests were all atomic bomb tests. 
 
 North Korea claimed that its January 2016 test was of a hydrogen bomb. 
 
 But experts cast doubt on the claim given the size of the explosion registered. 
 
 Plutonium or uranium? 
 
 Another question is what the starting material for the nuclear tests is. 
 
 Analysts believe the first two tests used plutonium, but whether the North used plutonium or uranium as the starting material for the 2013 test is unclear. 
 
 A successful uranium test would mark a significant leap forward in North Korea's nuclear programme. The North's plutonium stocks are finite, but if it could enrich uranium it could build up a nuclear stockpile. 
 
 Plutonium enrichment also has to happen in large, easy-to-spot facilities, whereas uranium enrichment can more easily be carried out in secrecy. 
 
 Can North Korea deliver it? 
 
 There is no consensus on exactly where North Korea is in terms of miniaturising a nuclear device so that it can be delivered via a missile. 
 
 According to that information leaked to the Washington Post, US intelligence officials now believe North Korea's claim that it has the technology to fit its missiles with nuclear warheads. 
 
 Japan's defence ministry has also recently stated this is a possibility. 
 
 The new assessment comes only weeks after North Korea tested what it said was an intercontinental ballistic missile (ICBM) capable of reaching the US mainland. While analysts doubt Pyongyang's claim, most experts agree the missile could reach Alaska or Hawaii. 
 
 What else do we know about the North's nuclear programme? 
 
 Image copyright EPA Image caption North Korea's nuclear actions have become a cause of concern among its neighbours, especially South Korea 
 
 A site in the mountains near Yongbyon, north of Pyongyang, is thought to be the North's main nuclear facility, while the January and September 2016 tests were said to have been carried out at the Punggye-ri site. 
 
 The Yongbyon site processes spent fuel from power stations and has been the source of plutonium for North Korea's nuclear weapons programme. 
 
 Both the US and South Korea have also said that they believe the North has additional sites linked to a uranium-enrichment programme. The country has plentiful reserves of uranium ore. 
 
 What has the global community done about this? 
 
 Image copyright Reuters Image caption The US and North Korea aimed at restarting denuclearisation talks before Kim Jong-il, right, died 
 
 The US, Russia, China, Japan and South Korea engaged the North in multiple rounds of negotiations known as six-party talks. 
 
 There were various attempts to agree disarmament deals with North Korea, but none of this has ultimately deterred Pyongyang. 
 
 In 2005, North Korea agreed to a landmark deal to give up its nuclear ambitions in return for economic aid and political concessions. In 2008, it destroyed the cooling tower at Yongbyon as part of the disarmament-for-aid deal. 
 
 But implementing the deal proved difficult and talks stalled in 2009. 
 
 Image copyright AFP Image caption North Korea carried a nuclear test in 2009, which resulted in seismic waves shown here in Seoul 
 
 The US never believed Pyongyang was fully disclosing all of its nuclear facilities - a suspicion bolstered when North Korea unveiled a uranium enrichment facility at Yongbyon, purportedly for electricity generation, to US scientist Siegfried Hecker in 2010. 
 
 In 2012, North Korea suddenly announced it would suspend nuclear activities and place a moratorium on missile tests in exchange for US food aid. But this came to nothing when Pyongyang tried to launch a rocket in April that year. 
 
 Image copyright AFP Image caption The United Nations has tightened sanctions against North Korea following nuclear tests 
 
 In March 2013, after a war of words with the US and with new UN sanctions over the North's third nuclear test, Pyongyang vowed to restart all facilities at Yongbyon. By 2015, normal operations there appeared to have resumed. 
 
 The 2016 tests brought international condemnation, including from China - the North's main trading partner, and only ally. 
 
 In 2017, the UN agreed a new sanctions package in response to the tests. 
 
 In August, US President Donald Trump threatened North Korea with "fire and fury" should the country not abandon its threats against the US. 
 
 The rhetoric from Washington failed, though, to rein in Pyongyang which in turn said it was developing a strike plan to hit the US territory of Guam, an island in the Western Pacific. ||||| Is Kim Jong-un’s order placing North Korea’s nuclear weapons ready for use at a moment's notice to carry out preemptive attacks realistic? Does North Korea even have an arsenal of nuclear weapons? Do North Korea’s long-range missile and nuclear tests mean they have the capability to place a small nuclear device on such a missile? 
 
 Interested in ? Add as an interest to stay up to date on the latest news, video, and analysis from ABC News. Add Interest 
 
 North Korea’s four nuclear tests and long-range missile tests that have placed a satellite into orbit are proof of some of its technical capabilities. But given the opaqueness of the North Korean regime, it’s difficult to get clear answers to those questions. 
 
 What is certain is that North Korea wants to develop those capabilities. 
 
 “North Korea is committed to developing a long-range, nuclear-armed missile that is capable of posing a direct threat to the United States,” said the Pentagon’s most recent assessment of North Korea’s weapons capabilities released a few weeks ago. 
 
 That same report said North Korea’s ballistic missile developments and nuclear weapons technology “are in line with North Korea’s stated objective of being able to strike the U.S. homeland.” But it appears North Korea has still not been able to achieve that capability. 
 
 How Many Nuclear Weapons Does North Korea Have? 
 
 The United States believes that North Korea has the capability to develop nuclear weapons, as shown by its four nuclear tests since 2006. However, it is difficult to pinpoint whether they have actually built a small arsenal of rudimentary nuclear weapons or have just built individual nuclear devices that are used for testing their technology. 
 
 U.S. intelligence has been vague in characterizing whether North Korea has nuclear weapons or not. What is known is that given their nuclear efforts with plutonium and uranium in the past decade, they have produced enough nuclear fuel to build some nuclear devices, but estimates vary among think tanks as to how many weapons they could build if they wanted to. 
 
 “It is generally estimated in open sources that North Korea has produced between 30 and 40 kilograms of separated plutonium, enough for at least half a dozen nuclear weapons,” according to a January 2016 Congressional Research Service report. 
 
 But other estimates range higher. The Institute for Science and International Security estimated in 2014 that North Korea could build 10 to 16 nuclear weapons. 
 
 The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies stated in a 2015 report that with continued North Korean nuclear production, “The stockpile increases from a current low level of 10 weapons to 20 weapons by 2020." 
 
 But the think tank estimated that potentially 100 weapons could be developed if North Korea ramped up its nuclear fuel production to maximum capability. 
 
 James Clapper, the Director of National Intelligence, told Congress in February that North Korea had restarted its plutonium reactor in Yongbyon that had been shut down in 2007. 
 
 “We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor’s spent fuel within a matter of weeks to months,” Clapper said. That means North Korea will soon have more nuclear fuel that could be used for nuclear weapons. 
 
 Can North Korea Place a Small Nuclear Weapon on an ICBM? 
 
 As part of its commitment to threaten the United States with an intercontinental ballistic missile, North Korea has a weapon called the KN-08, a long-range missile that can be launched from mobile platforms that can be hard to detect, Clapper noted. 
 
 The KN-08 is more worrisome to U.S. defense officials than the stationary rocket launches that North Korea has used to conduct what it says are satellite test launches because it can be hard to detect mobile platforms on short notice. 
 
 "We assess that North Korea has already taken initial steps toward fielding this system, although the system has not been flight-tested," Clapper said. 
 
 In April 2013, an assessment by the Defense Intelligence Agency (DIA) was made public at a congressional hearing that indicated North Korea might be capable of placing a nuclear weapon small enough to be delivered by a missile. 
 
 "DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles. However, the reliability will be low," the unclassified assessment noted. Moderate confidence is an intelligence term that signals plausibility. 
 
 The DIA analysis led to push-back from the Pentagon and Clapper, who said it was only a DIA assessment and that the intelligence community as a whole had not made that determination. 
 
 “It would be inaccurate to suggest that the North Korean regime has fully developed and tested the kinds of nuclear weapons referenced in the passage," said then-Pentagon spokesman George Little. “I would add that the statement read by the Member is not an Intelligence Community assessment. Moreover, North Korea has not yet demonstrated the full range of capabilities necessary for a nuclear-armed missile.” ||||| WASHINGTON (Reuters) - The U.S. military detected a failed North Korean test launch of an intermediate ballistic missile, the Pentagon said on Saturday. 
 
 FILE PHOTO - A North Korean flag flies on a mast at the Permanent Mission of North Korea in Geneva October 2, 2014. REUTERS/Denis Balibouse/File Photo 
 
 The test-firing was the latest in a series conducted by North Korea in violation of United Nations resolutions, drawing a fresh U.S. vow to take its concerns to the world body and seek to hold North Korea “accountable for these actions.” 
 
 The U.S. Strategic Command’s systems detected that the Musudan missile failed in a launch conducted near North Korea’s northwestern city of Kusong, the Pentagon said in a statement. 
 
 It provided no details on what went wrong but said the launch never posed a threat to North America. 
 
 “We strongly condemn this and North Korea’s other recent missile tests,” said U.S. Navy Commander Gary Ross, a Pentagon spokesman. “Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, is ironclad. We remain prepared to defend ourselves and our allies from any attack or provocation.” 
 
 South Korea’s military said early on Sunday that North Korea fired what it believed was a Musudan missile at 0333 GMT on Saturday (11:33 p.m. EDT on Friday) but it failed immediately after launch. It did not elaborate on the reason for the failure. 
 
 “The North’s ballistic missile launch is a clear violation of U.N. Security Council resolutions and we strongly condemn the North’s illegal act of provocation,” the South’s office of the Joint Chiefs of Staff said in a statement. 
 
 Ross said the United States called on North Korea to “refrain from actions that further raise tensions in the region.” 
 
 The top U.S. diplomat for East Asia said late last month that Washington would speed up deployment of the U.S. Terminal High Altitude Area Defense (THAAD) anti-missile system to South Korea given the pace of North Korea’s missile tests, and it would be stationed there “as soon as possible.” 
 
 South Korea and the United States have agreed to deploy the anti-missile system in the South to counter the North’s threat. China, the North’s main diplomatic ally, has opposed the move saying it would destabilize regional security balance, but Washington has said the system was not aimed at China. 
 
 Daniel Russel, assistant secretary of state for East Asia, also told a congressional hearing the United States was in discussions with international partners, including the European Union, to deny North Korea access to international banking infrastructure after its recent nuclear and missile tests. 
 
 Pyongyang is already under heavy international sanctions over its missile and nuclear tests. North Korea conducted its fifth nuclear blast on Sept. 9.
– US defense agencies detected an attempted missile launch in western North Korea late Friday, near the city of Kusong, Reuters reports. The object—believed to be a Musudan intermediate-range ballistic missile—failed immediately after launch, per South Korea's military. American and South Korean defense agencies declined to provide any further details, but both governments condemned the act as unprovoked aggression. The missile test is the first since North Korea's latest underground nuclear test, which took place on Sept. 9. While it's murky whether North Korea actually has nuclear warheads, so far it hasn't verifiably demonstrated the technical capability, even if it does have them, to mount those weapons on long-range missiles that could strike the US mainland. The missile launch follows a week of joint US-South Korean war games, which the North has called a dry run for an invasion, Stars and Stripes reports. The presence of the USS Ronald Reagan aircraft carrier, which arrived in Busan, South Korea, hours before the failed missile launch, lends a further show of muscle to repeated US commitments to defend South Korea against northern aggression.