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) ||||| North Korea says it successfully tested a hydrogen nuclear bomb, a development that, if true, represents a dramatic advance in its military capabilities. Previously, the nation has tested plutonium weapons in three underground nuclear tests, but hydrogen bombs are much more powerful. 
 
 
 
 Around 10 a.m. local time on Wednesday, an earthquake was detected near North Korea's nuclear test site, sparking concern that the nation had conducted a fourth nuclear test. A short time later, a statement was read on North Korean state TV claiming that the nation detonated a "miniaturized" hydrogen nuclear bomb, and the test was a "perfect success." "If there's no invasion on our sovereignty we will not use nuclear weapon," the statement said. "This H-bomb test brings us to a higher level of nuclear power." 
 
 Statements from North Korea are notoriously untrustworthy — last summer the nation claimed to have invented a vaccine for Ebola, HIV, and "a number of cancers" — but obviously this is a concerning development. Here's what we know so far. 
 
 Did North Korea actually test a hydrogen bomb? 
 
 It seems highly likely that North Korea conducted some kind of nuclear test, but no one has independently verified that the weapon tested was in fact a hydrogen bomb — or, as North Korea described it, an "H-bomb of justice." And, experts are massively doubtful about the whole thing. 
 
 Or, as White House spokesperson Josh Earnest put it on Wednesday afternoon, there is “significant and understandable skepticism of the claims of the North Korean regime.” 
 
 The U.S. Geological Survey measured Wednesday morning's seismic activity, which took place northeast of Sungjibaegam, at a magnitude of 5.1, and South Korean experts said it appeared the quake was caused by a man-made explosion. A 4.9-magnitude earthquake was recorded near the same spot before North Korea confirmed its last nuclear test in 2013. 
 
 #NEWSGRAPHIC Map locating a quake in North Korea near its nuclear test site pic.twitter.com/q4WgE0v0H8 — Agence France-Presse (@AFP) January 6, 2016 
 
 People were evacuated from buildings after tremors from the explosion were felt in the Chinese border town of Yanji: 
 
 Picture shows students evacuated in Yanji near China-#DPRK border after Pyongyang's H-bomb test pic.twitter.com/kmSGvV5MJ9 — CCTVNEWS (@cctvnews) January 6, 2016 
 
 Recent satellite photos analyzed by 38 North, a Washington research institute that follows North Korea’s nuclear activity, showed evidence that the nation has been digging a new tunnel at its nuclear test site. Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies in Monterey, California, said last month that the tunnel made it “more likely that they will conduct a test in the coming year.” But he told the Washington Post that Wednesday’s blast wasn’t drastically bigger than in previous tests, suggesting it wasn’t a hydrogen bomb. 
 
 
 
 The South Korean defense ministry said it can't confirm what kind of event occurred, and a senior U.S. administration official told CNN that it may take days to obtain scientific data that can determine whether the test was successful. 
 
 South Korean lawmaker Lee Cheol Woo told the AP that he was told that the seismic wave caused by the test was small enough that people were skeptical that it could have been caused by even a failed hydrogen-bomb test. 
 
 For what it's worth, here's the announcement that aired on North Korean TV: 
 
 Why is this different from North Korea’s previous weapons tests? 
 
 In the past, North Korea has only tested bombs that use fission to break the atomic nucleus of elements like plutonium, releasing large amounts of energy. A thermonuclear bomb uses fusion to combine small atoms, such as hydrogen, creating a much larger blast. A fission weapon typically yields around ten kilotons, while a fusion weapon is measured in megatons (1,000 kilotons). If North Korea has a true hydrogen weapon, it would be hundreds of times more powerful than the bombs dropped on Hiroshima and Nagasaki. 
 
 North Korea’s development of a hydrogen bomb is also concerning because fusion weapons are much harder to make. Some experts believe Pyongyang may already have 10 to 15 atomic weapons, but the bombs are rudimentary. In May, it claimed to have developed miniaturized nuclear weapons, which could be delivered via missile, but many were skeptical. As the Associated Press notes, a successful test of a hydrogen bomb would mean that North Korean engineers are much closer to their stated goal of creating a stockpile of nuclear-tipped missiles that can reach the U.S. 
 
 North Korean leader Kim Jong-un claimed last month that his scientists were developing a hydrogen weapon, but Bruce Bennett, a senior defense analyst at the nonprofit Rand Corp., suggests that he may only have "boosted" weapons. As Bennett explains in a CNN opinion piece, that’s a far more plausible possibility, considering that North Korea only conducted its first nuclear test in 2006: 
 
 En route to the development of fusion weapons, some countries develop so-called "boosted" weapons, which use a small amount of fusion to boost the fission process, causing more large atoms to fission and thus releasing more energy — initially, perhaps a weapon of 50 kilotons or so. Because some fusion is involved in such a weapon, Kim may be claiming that he has achieved a hydrogen bomb when in practice he only has a boosted weapon. 
 
 
 
 What difference would a boosted nuclear weapon make? If North Korea really has a boosted nuclear weapon of perhaps 50 kilotons, it could do significant damage in a city as densely populated as Seoul, South Korea: About 250,000 people could be killed in such a strike, or about 2.5% of the population. This would mark a genuine advance in the level of damage that North Korea is capable of doing. And if North Korea one day produces a true hydrogen bomb of, say, one megaton yield, then it would be deadlier still. 
 
 Bennett added to the BBC, "The bang they should have gotten would have been 10 times greater than what they're claiming. So Kim Jong-un is either lying, saying they did a hydrogen test when they didn't, they just used a little bit more efficient fission weapon — or the hydrogen part of the test really didn't work very well or the fission part didn't work very well." 
 
 How are other nations responding? 
 
 The South Korean government convened an emergency meeting of its National Security Council. "This is a grave provocation to our security, threatening the survival and future of our nation and further directly challenging peace and stability in the world," said South Korean president Park Geun-hye. "The government — in close cooperation with the international community — should have North Korea pay a price without fail for the latest nuclear test." 
 
 Regardless of whether North Korea tested an H-bomb or conducted a smaller nuclear test, the world is very mad at Kim Jong-un. 
 
 Several other nearby nations held emergency meetings as well, and Japanese prime minister Shinzō Abe issued a statement condemning the test. "The nuclear test that was carried out by North Korea is a serious threat to the safety of our nation and we absolutely cannot tolerate this," he said. "This clearly violates U.N. Security Council resolutions and is a grave challenge against international efforts for non-proliferation.” 
 
 Although China is an ally, they were also not impressed by North Korea's bragging — and were not warned about the test in advance. A spokesperson at the Chinese Ministry of Foreign Affairs told reporters, “China is strongly against this act.” North Korea's only friend is probably getting pretty incensed at this point. As The Guardian points out, "Kim has refused to rejoin the Chinese-led six-party nuclear talks. In September, he snubbed President Xi Jinping’s invitation to attend celebrations marking the end of the second world war." 
 
 The New York Times adds that it was never surprising that North Korea was far less interested in taking part in international nuclear agreements than Iran: 
 
 From Pyongyang’s viewpoint, there is little incentive to give up the nuclear arsenal. The world is not exactly banging on North Korea’s door to do business the way it wants to resume relations with Iran: The North has no oil, no striving middle class, and little strategic value in the modern world. Its greatest power is the threat it poses to one of the most prosperous corners of the globe. 
 
 One analyst at the New Zealand Contemporary China Research Centre at the Victoria University of Wellington told the Washington Post, “In a way, this is a protest against Beijing. They are saying: ‘we can do whatever we want. This shows our independence and we don’t need your approval.’” 
 
 Russia wants everyone to chill out for a minute, in case North Korea didn't actually test a hydrogen bomb. The foreign ministry called on "all interested sides to preserve maximum restraint and to not take actions that could rouse the uncontrolled growth of tensions in Northeast Asia," per the AP. 
 
 The United Nations Security Council said it would meet on Wednesday, at the request of Japan and the United States. The 15-nation council put North Korea under sanctions after its first nuclear test, and if a fourth nuclear test is confirmed it's likely the penalties will be increased. 
 
 The White House National Security Council released this statement: 
 
 We are aware of seismic activity on the Korean Peninsula & have seen claims of a nuclear test. Statement: pic.twitter.com/WvyFkpJcZo — WH National Security (@NSCPress) January 6, 2016 
 
 State Department spokesperson John Kirby added, according to the Washington Post, “We have consistently made clear that we will not accept it as a nuclear state. We will continue to protect and defend our allies in the region, including [South] Korea, and will respond appropriately to any and all North Korean provocations.” 
 
 Marco Rubio was the first GOP presidential candidate to weigh in, offering this predictable assessment: 
 
 who has been expanding his nuclear arsenal while President Obama has stood idly by. (2/5) — Marco Rubio (@marcorubio) January 6, 2016 
 
 If this test is confirmed, it will be just the latest example of the failed Obama-Clinton foreign policy. (3/5) — Marco Rubio (@marcorubio) January 6, 2016 
 
 Senator Ted Cruz also blamed Obama and Hillary Clinton for the nuclear test, and then drew his own conclusions for what this all meant for Iran. "We saw last night the consequences of this deal if it is allowed to go forward," he said, according to the Washington Post. "[Iran's] nuclear test may not be underground, measured by an earthquake. It may be above the skies of Tel Aviv, or New York, or Los Angeles." 
 
 Donald Trump later added that North Korea was a problem that China should deal with. He also offered his analysis of Kim Jong-un, calling him a "madman." 
 
 "And nobody talks to him, other than of course Dennis Rodman. That's about it." 
 
 Hillary Clinton responded to the incident by drawing eyes back to her résumé. "As secretary, I championed the United States' pivot to the Asia Pacific — including shifting additional military assets to the theater — in part to confront threats like North Korea and to support our allies," she said in a statement. 
 
 Why would North Korea conduct a test now? 
 
 Sussing out Kim’s motives is nearly impossible, but we do have some clues about the timing. Friday is the leader’s 33rd birthday, and new nuclear capabilities are the perfect gift for a dictator. It’s also possible that Kim wants a major accomplishment to brag about in May, when the Korean Workers’ Party holds its first Communist Party congress in 36 years. 
 
 Phil Robertson at Human Rights Watch thought these were lousy reasons to conduct a test.“Kim Jong-un may think it appropriate to celebrate his birthday early with a nuclear test, but even a hydrogen bomb should not cause the world to forget that the Kim family’s hereditary dictatorship is built on the systematic brutalization and abuse of the North Korean people,” he said in a statement. 
 
 Kim may also be anxious about North Korea’s shaky ties to China, its last major ally, though the move is likely to worsen their relationship even further. Spokesperson Hua Chunying said China “firmly opposes” the purported hydrogen-bomb test, adding, ”North Korea should stop taking any actions which would worsen the situation on the Korean peninsula.” 
 
 North Korea offered a poetic explanation for its latest act of defiance. The televised statement said that for North Korea to give up its nuclear weapons program while the U.S. continues its "hostile policy" would be "as foolish as for a hunter to lay down his rifle while a ferocious wolf is charging at him." According to Shanghaiist, the state media agency, KCNA, added that any nuclear weapons tested were "self-defense against the US having numerous and humongous nuclear weapons." 
 
 KCNA also noted that "The U.S. is a gang of cruel robbers which has worked hard to bring even a nuclear disaster to the DPRK, not content with having imposed the thrice-cursed and unheard-of political isolation, economic blockade and military pressure on it for the mere reason that it has differing ideology and social system and refuses to yield to the former's ambition for aggression." 
 
 The L.A. Times talked to Bonnie Glaser at the Center for Strategic and International Studies, who "recalled one official saying that U.S. bargaining in the past had taught Pyongyang that it was 'OK to pee on the rug' and what was necessary now was to not get hysterical every time they 'engage in bad behavior' by going back with a fresh package of proposals." 
 
 Regardless of what the next year brings — does the U.S. try to talk it out with Kim? — it seems clear that North Korea has invited the world to invent new ways to sanction it, which will give the country plenty more reasons to hate the rest of the world. 
 
 This post has been updated throughout. ||||| 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.
– 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."