Experts cast doubt on claim, saying evidence points to test involving uranium or plutonium device, while UN security council will discuss possible sanctions 
 
 North Korea has conducted its fourth nuclear test, the country’s state media has announced, in a move that could take it a step closer to developing nuclear warheads capable of striking the US mainland. 
 
 
 
 Some experts, though, said initial evidence was pointing towards a test involving a uranium or plutonium device and not, as Pyongyang has claimed, a far more powerful hydrogen bomb. 
 
 North Korea to make 'major announcement' following artificial earthquake near nuclear test site – live Read more 
 
 An announcement on North Korean television said the country had successfully tested a “miniaturised hydrogen bomb” underground on Wednesday morning, describing it as an “act of self-defence” against the US. North Korean claims about the size and type of bomb have not been independently verified. 
 
 If the claims are true, it would be the first time the North has successfully tested a hydrogen bomb and could also enable the secretive state to launch long-range nuclear missiles. 
 
 “If it’s true, it means they’ve made something smaller scale, capable of being put onto a missile, said John Carlson, the former head of the Australian Safeguards and Non-Proliferation Office. “I think we can assume the previous tests they’ve carried out have been devices too large to fit onto a missile.” 
 
 South Korea’s spy agency believes the seismic wave reported was more likely caused by an atomic bomb, Lee Cheol Woo, a South Korean politician, said. 
 
 Yang Uk, a senior research fellow at the Korea Defence and Security Forum, said: “Given the scale it is hard to believe this is a real hydrogen bomb. They could have tested some middle stage kind (of device) between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim.” 
 
 The North Korean state television broadcast said the test was a “complete success” which propelled the country into the “rank of advanced nuclear states”. It had been overseen by the country’s leader, Kim Jong-un and conducted ahead of his 33rd birthday this Friday. 
 
 
 
 “[North Korea’s] fate must not be protected by any forces, only by [North Korea] itself,” the broadcast said. North Korea would not abandon its nuclear weapons program as long as the US maintained what it called “its stance of aggression”. 
 
 “The US has gathered forces hostile to DPRK [Democratic People’s Republic of Korea] and raised a slanderous human rights issue to hinder DPRK’s improvement,” it said. 
 
 “It is just to have H-bomb as self-defence against the US having numerous and humongous nuclear weapons. The DPRK’s fate must not be protected by any forces but DPRK itself.” 
 
 The United Nations security council was planning to meet on Wednesday morning in New York to discuss North Korea’s nuclear test, council diplomats said. 
 
 The White House said it could not confirm North Korea’s claims, but added the United States would respond appropriately to provocations and defend its allies. The EU said the test was a “grave violation” of UN resolutions. 
 
 Phillip Hammond, the British foreign secretary, said on a visit to Beijing: “I think I can say that Britain and China are pretty much completely aligned on North Korean nuclear. We both strongly oppose the acquisition or testing of nuclear weapons by North Korea and we both want to see a resumption of the six party talks. 
 
 “So far as I think any of us can tell, there is no reason to doubt what the North Koreans are claiming – that they have carried out a test detonation of an H bomb. It is clearly very bad news for the non-proliferation agenda generally and for security in the region specifically.” 
 
 South Korean president Park Geun-hye said the country would take decisive measures against any additional provocations by North Korea and work with the international community to make sure the country pays the price for its latest nuclear test. 
 
 Park said the latest test could change the fundamental nature of the situation over North Korea’s nuclear programme. 
 
 Prime minister Shinzo Abe said Japan would make a firm response to North Korea’s challenge against nuclear non-proliferation, calling its test a threat to Japan’s security. 
 
 
 
 The test is bound to ratchet up tensions between the isolated country and its neighbours as well as Washington. China’s foreign ministry said that Beijing did not have advance knowledge of the test and added that it firmly opposes Pyongyang’s action. 
 
 
 
 North Korea conducted nuclear tests in 2006, 2009 and 2013, drawing condemnation and rounds of UN sanctions banning trade and financing activities that aid its weapons program. 
 
 
 
 A successful fourth test could deepen North Korea’s international isolation if the UN security council decided to respond with more sanctions. Pyongyang is thought to have developed several crude nuclear weapons. 
 
 Timeline: North Korea’s nuclear weapons development Read more 
 
 Since becoming North Korean leader in late 2011, Kim Jong-un has stepped up condemnation of joint US-South Korean military drills he believes are preparation for an invasion. 
 
 
 
 In early 2013, Kim responded to UN sanctions imposed after North Korea’s third nuclear test by unleashing a barrage of threats aimed at South Korea, the US and Japan. 
 
 Kim appears to have caught the world off-guard yet again. He did not mention his country’s pursuit of a nuclear deterrent in his New Year’s address, an omission some analysts say was designed to improve the prospects of a summit with the Chinese leader, Xi Jinping. 
 
 Last month Washington dismissed Kim’s claims that his country had developed a hydrogen bomb – a development that, if true, would mark significant progress in North Korea’s nuclear capability. Hydrogen bombs are capable of producing a far more powerful blast than basic atom bombs. 
 
 An intensity shake map released by the US Geological Survey on 5 January shows the location where a preliminary 5.1 magnitude earthquake struck in North Korea. Photograph: US Geological Survey/EPA 
 
 John Carlson said the veracity of North Korea’s claim would be difficult to determine “until we can analyse any gases emitted, and it could take several days for those to show up, if it all”. 
 
 He said it would be a “major challenge” for the North Korean regime to develop a thermonuclear weapon, particularly a miniaturised version, which required even greater expertise. 
 
 “Their claim would be far more convincing if it was a larger size,” he said. 
 
 Carlson, who headed Australia’s safeguards office for more than two decades, said it was possible Pyongyang had actually produced a lower-yield “boosted explosion”, in which the hydrogen isotope tritium undergoes partial fusion, allowing them to describe the device as a hydrogen bomb. 
 
 “I think it’s feasible that they’ve done that and they’re claiming that they’ve done more,” he said. 
 
 Speculation that the regime in Pyongyang had conducted a surprise nuclear test on Wednesday rose after seismologists from South Korea, China and other countries said they had detected a “manmade” earthquake in the country that could have been caused by an explosion. 
 
 
 
 South Korean meteorological officials said the epicentre of the quake was 49km (30 miles) from the Punggye-ri site in North Hamgyong province, where North Korea has conducted nuclear tests in the past. 
 
 How to stop worrying and embrace a nuclear North Korea Read more 
 
 Yonhap news agency in South Korea said North Korea had not given notice of the test, which was carried out at 10am local time (1am GMT), to the US or its traditional ally China. 
 
 Wednesday’s tremor was a “suspected explosion”, the China earthquake network centre said on its website. It gave the magnitude as 4.9. 
 
 The US Geological Service said it had detected a 5.1 magnitude quake, and the European Mediterranean seismological centre said it had also detected unusual seismic activity in North Korea. 
 
 “We suspect a manmade earthquake and are analysing the scale and epicentre of the quake with the geoscience and mineral resource institute of South Korea,” a Korea meteorological administration official said in the South Korean capital, Seoul. ||||| North Korean leader Kim Jong Un gives a new year’s address for 2016 in Pyongyang, in this photo released Friday. (Kyodo/Reuters) 
 
 North Korea claimed Wednesday that it had successfully tested a hydrogen bomb, a claim that, if true, would mark a huge step forward in its nuclear capability. 
 
 “We’ve carried out a hydrogen bomb test,” a newsreader on the state-run Korean Central Television station announced in a special broadcast from Pyongyang. 
 
 “North Korea was forced to develop its nuclear arsenal because of the U.S.’s hostile policy against North Korea,” she said. “However, as a peaceful nation and a nuclear powered-nation, North Korea will be a responsible state and will not use its nuclear power before [an attack] and will not transfer the technology to others.” 
 
 There was some skepticism about the claim, with nuclear experts noting that the yield appeared to be similar to North Korea’s three previous atomic tests, rather than the “enormous” yield that would be expected if it had been a thermonuclear test. 
 
 “We are aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site and have seen Pyongyang’s claims of a nuclear test,” said John Kirby, a spokesman at the State Department. “We are monitoring and continuing to assess the situation in close coordination with our regional partners.” 
 
 
 
 
 
 Either way, Pyongyang’s provocative action will present a new challenge to the outside world, which has struggled to find ways to end North Korea’s nuclear defiance. 
 
 “North Korea’s fourth test — in the context of repeated statements by U.S., Chinese, and South Korean leaders — throws down the gauntlet to the international community to go beyond paper resolutions and find a way to impose real costs on North Korea for pursuing this course of action,” said Scott Snyder, a Korea expert at the Council on Foreign Relations. 
 
 Kim Jong Un’s regime hinted in December that it had built a hydrogen bomb to “defend its sovereignty and the dignity of the nation.” Some analysts were doubtful, saying the young leader appeared primarily concerned with trying to bolster his legitimacy. 
 
 But on Wednesday, North Korea said in a special broadcast that it had carried out a “successful” hydrogen bomb test. “With this hydrogen bomb test, we have joined the major nuclear powers,” the newsreader said. 
 
 Hydrogen, or thermonuclear, bombs are exponentially more powerful and destructive than atomic devices. An atomic bomb uses fission to break up the atomic nucleus and release energy, while a hydrogen or thermonuclear bomb uses fusion to add to the nucleus. This leads to an enormous explosion resulting from an uncontrolled, self-sustaining chain reaction. 
 
 Kim has repeatedly asserted North Korea’s status as a nuclear-armed country and has resolutely refused to return to multilateral talks aimed at persuading it to abandon its nuclear weapons program. 
 
 North Korea had conducted three nuclear tests since 2006 but only one during Kim’s reign, in February 2013.To the surprise of many analysts, there had been no fourth test. 
 
 Then, there were signs of unusual seismic activity around North Korea’s main nuclear test site Wednesday morning, sparking fears that Pyongyang ordered the detonation of another atomic device two days before Kim’s birthday. 
 
 “We have consistently made clear that we will not accept [North Korea] as a nuclear state,” Kirby said. “We will continue to protect and defend our allies in the region, including the Republic of Korea, and will respond appropriately to any and all North Korean provocations.” 
 
 Earthquake agencies in China, Japan and the United States all recorded unusual seismic activity in the northeastern corner of North Korea at 10 a.m. local time Wednesday. 
 
 The U.S. Geological Survey recorded a shallow 5.1-magnitude quake about 20 miles from the facility at Punggye-ri, where North Korea has carried out its three previous nuclear tests. Japan’s Meteorological Agency said that it appeared to be some kind of artificial explosion and that the waveform was very similar to the ones detected at the past nuclear tests, public broadcaster NHK reported. 
 
 Many analysts have been surprised that such a long period has passed without another test, because it is by testing that North Korea can advance its program. 
 
 “I think they have a technological path in mind,” said Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies in Monterey, Calif. 
 
 In December, Lewis noted that satellite pictures showed North Korea appeared to be building a new tunnel at its nuclear test site, warning that the Pyongyang regime might be preparing to conduct a fourth atomic test. “There is a lot of tunneling at the test site, which could mean they have a bunch of tests planned,” he said. 
 
 Although analysts were still awaiting more data, Lewis said that Wednesday’s explosion looked very similar to past tests and was not enormous, suggesting it was not a hydrogen bomb. 
 
 In Seoul and Tokyo, the governments called emergency national security meetings to discuss the possibility of a nuclear test. 
 
 “This nuclear test by North Korea is a major threat to our country’s security, and I absolutely cannot accept it,” Japanese Prime Minister Shinzo Abe told reporters in Tokyo on Wednesday. “Also, it is clearly a violation of United Nations Security Council resolutions so . . . we will take strong measure, including steps within the U.N. Security Council.” 
 
 Joel Wit, a former U.S. diplomat who runs the 38 North website dedicated to North Korea, said that the purpose of the test remained unclear. 
 
 “What is clear is that North Korea is moving forward with its nuclear weapons program and that the United States, China and the international community need to come up with more effective ways to deal with this growing threat,” he said. 
 
 Yoonjung Seo in Seoul and Yuki Oda in Tokyo contributed to this report. ||||| SEOUL, South Korea (AP) — North Korea said Wednesday it had conducted a powerful hydrogen bomb test, a defiant and surprising move that, if confirmed, would be a huge jump in Pyongyang's quest to improve its still-limited nuclear arsenal. 
 
 People watch a TV news program showing North Korea's announcement, at the Seoul Railway Station in Seoul, South Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb... (Associated Press) 
 
 People watch a TV news program showing North Korea's announcement, at the Seoul Railway Station in Seoul, South Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb... (Associated Press) 
 
 In this Oct. 10, 2015, file photo, North Korean leader Kim Jong Un delivers remarks at a military parade in Pyongyang, North Korea. North Korea said on Wednesday, Jan. 6, 2016, it has conducted a hydrogen... (Associated Press) 
 
 People watch a TV news program showing North Korea's announcement, at the Seoul Railway Station in Seoul, South Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb... (Associated Press) 
 
 North Koreans watch a news broadcast on a video screen outside Pyongyang Railway Station in Pyongyang, North Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb... (Associated Press) 
 
 In this image made from video from KRT, the North Korean state broadcaster, a North Korean woman speaks during a broadcast aired on Wednesday, Jan. 6, 2016. North Korea says it has conducted a hydrogen... (Associated Press) 
 
 North Koreans react as they watch a news broadcast on a video screen outside Pyongyang Railway Station in Pyongyang, North Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a... (Associated Press) 
 
 North Koreans watch a news broadcast on a video screen outside Pyongyang Railway Station in Pyongyang, North Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb... (Associated Press) 
 
 People walk by a screen showing the news reporting about an earthquake near North Korea's nuclear facility, in Seoul, South Korea, Wednesday, Jan. 6, 2016. South Korean officials detected an "artificial... (Associated Press) 
 
 A television anchor said in a typically propaganda-heavy statement that the North had tested a "miniaturized" hydrogen bomb, elevating the country's "nuclear might to the next level" and providing it with a weapon to defend against the United States and its other enemies. 
 
 The statement said the test was a "perfect success," and the announcement was celebrated on the streets of Pyongyang. 
 
 South Korean President Park Geun-hye ordered her military to bolster its combined defense posture with U.S. forces and called the test a "grave provocation" and "an act that threatens our lives and future." 
 
 There has long been skepticism by Washington and nuclear experts about past North Korean claims about H-bombs, which are much more powerful, and much more difficult to make, than atomic bombs. A confirmed test, however, would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations, which quickly announced an emergency Security Council meeting on North Korea. It would also further worsen already abysmal relations between Pyongyang and its neighbors. 
 
 North Korean nuclear tests catch global attention because each new blast is seen as pushing North Korea's scientists and engineers closer to their goal of building a bomb small enough to place on a missile that can reach the U.S. mainland. 
 
 A successful H-bomb test would be a big step for the North, and the announcement prompted skepticism. Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic weapons that use fission. In a hydrogen bomb, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity. 
 
 Writing in December, after North Korean leader Kim Jong Un bragged of H-bomb capabilities, nuclear expert Jeffrey Lewis noted that building such a bomb "would seem to be a bit of a stretch for the North Koreans." 
 
 But, he wrote on the North Korea-focused 38 North website, "The North has now had a nuclear weapons program for more than 20 years. This program has yielded three nuclear tests. North Korean nuclear scientists have access to their counterparts in Pakistan, possibly Iran and maybe a few other places. We should not expect that they will test the same fission device over and over again." 
 
 One expert in Seoul said the 5.1 magnitude of the quake set off by Wednesday's test was likely too small to be an H-bomb test. However, the North could have experimented with a "boosted" bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel, said Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University. 
 
 In Pyongyang, meanwhile, the announcement was greeted with an expected rush of nationalistic pride, and some bewilderment. 
 
 A large crowd gathered in front of the city's main train station to watch the special bulletin announcing the news of the test on a big screen in the station's plaza. Some took videos or photos of the screen on their mobile phones. Others applauded and cheered as the TV announcer, wearing a pink traditional Korean gown, announced the news. 
 
 One of the people in the plaza, Kim Sok Chol, 32, told The Associated Press that he doesn't know much about H-bombs, but added that "Since we have it the U.S. will not attack us." 
 
 University student Ri Sol Yong, 22, said, "If we didn't have powerful nuclear weapons, we would already have been turned into the slaves of the U.S." 
 
 Just how big a threat North Korea's nuclear program currently poses to the outside world is something of a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs. 
 
 Some analysts say the North hasn't likely achieved the technology needed to manufacture a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But there is a growing debate on just how far the North has advanced in its secretive nuclear and missile programs. 
 
 North Korea needs nuclear tests for practical military reasons. To build a credible nuclear program, the North must explode new nuclear devices — including miniaturized ones — so its scientists can continually improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents, and diplomatic bargaining chips, against its enemies — and especially against the United States, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War. 
 
 Strong condemnation of the announced test rolled in from around the world, including from Lassina Zerbo, the head of the Vienna-based CTBTO, which has a worldwide network of monitoring stations to detect nuclear testing. Zerbo told AP by phone, "This is indeed a wakeup call ... and I am convinced it will have repercussions on North Korea and international peace and stability." 
 
 On Wednesday, the first indication of a possible test came when the U.S. Geological Survey measured an earthquake with a magnitude of 5.1. An official from the Korea Metrological Administration, South Korea's weather agency, said the agency believed the earthquake was caused artificially, based on an analysis of the seismic waves; it originated 49 kilometers (30 miles) north of Kilju, the northeastern area where North Korea's main nuclear test site is located. The country conducted all three previous atomic detonations there. 
 
 The announcement came as a surprise. 
 
 North Korea hadn't conducted a nuclear explosion since early 2013, and leader Kim Jong Un did not mention the country's nuclear weapons in his New Year's speech. Some outside analysts speculated that Kim was worried about deteriorating ties with China, the North's last major ally, which has shown signs of greater frustration at provocations and a possible willingness to allow stronger U.N. sanctions. 
 
 Pyongyang says its nuclear weapons program is necessary to defend itself against the United States. North Korea under Kim Jong Un has pledged to bolster its nuclear arsenal unless Washington scraps what Pyongyang calls a hostile policy. 
 
 Washington sees North Korea's pursuit of nuclear weapons and ballistic missiles as a threat to world security and to its Asian allies, Japan and South Korea. 
 
 The Korean Peninsula remains technically at war, as the 1950-53 conflict ended in an armistice. Washington stations more than 28,000 troops in South Korea as a buttress against any North Korean aggression. Tens of thousands more are in nearby Japan. 
 
 The test announcement comes amid failed diplomatic efforts to persuade the North to give up its nuclear ambitions. Six-nation negotiations on dismantling North Korea's nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim Jong Un's father, Kim Jong Il, who died in late 2011. 
 
 ___ 
 
 AP writer Hyung-jin Kim in Seoul and George Jahn in Vienna contributed to this story. 
 
 Follow Foster Klug on Twitter: @APKlug
– A tense 2016 lies ahead on the Korean Peninsula after a North Korean nuclear test that could mark an alarming leap forward in the country's technology. Pyongyang claims that it successfully detonated a hydrogen bomb for the first time, though experts say the Wednesday morning explosion— believed to have caused a 5.1-magnitude quake—was closer in size to North Korea's three previous nuclear tests than to a hydrogen bomb explosion, reports the Washington Post. Kim Jong Un claimed last month that the country had an H-bomb, a weapon much more powerful and harder to make than an atomic bomb, and a confirmed test would almost certainly lead to tough new sanctions against North Korea, the AP reports. North Korean state media reported that the test of the "miniaturized" H-bomb was a "perfect success." It could be weeks before an H-bomb test can be confirmed by outside experts, who seem skeptical of Pyongyang's claims. "Given the scale, it is hard to believe this is a real hydrogen bomb," a rep from the Korea Defense and Security Forum tells Reuters. "They could have tested some middle stage kind [of device] between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim." The Australian Broadcasting Corporation reports that the United Nations Security Council plans to hold an emergency meeting Wednesday to discuss the nuclear test. (At the start of 2016, Kim said he was ready for war.)