SEOUL/WASHINGTON North Korea said it successfully tested a miniaturised hydrogen nuclear bomb on Wednesday, but atomic weapons experts and U.S. government agencies cast doubt on the isolated nation's ability to make such an advance in its arsenal.
The test, the fourth time that North Korea has exploded anuclear device, unnerved South Korea and Japan and drewworld criticism, including from China and Russia, Pyongyang's two main allies.
U.N. Secretary-General Ban Ki-moon condemned NorthKorea's action, calling it "profoundly destabilizing forregional security," while U.S. House Speaker Paul Ryan said it"looks like a provocation".
North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006 and could faceadditional measures. The Security Council was holding an emergency meeting to weigh what steps it could take.
The explosion caused an earthquake that was measured by the United States Geological Survey. The nuclear test was ordered by leader Kim Jong Un and successfully conducted at 10 a.m. local time (0130 GMT), North Korea's official KCNA news agency said.
"Let the world look up to the strong, self-reliantnuclear-armed state," Kim wrote in what North Korean state TVdisplayed as a handwritten note.
U.S. government experts do not believe the device was a hydrogen bomb, U.S. government sources said. It likely will take several days to determine more precisely what kind of nuclear device Pyongyang set off as a variety of sensors, including "sniffer planes," collect evidence.
South Korean intelligence officials and several analystsalso questioned whether Wednesday's explosion was a test of afull-fledged hydrogen device, pointing to the fact that it wasroughly as powerful as North Korea's last atomic test in 2013.
Stocks across the world fell for a fifth consecutive day as the North Korea tension added to a growing list of geopolitical worries and China fuelled fears about its economy by allowing the yuan to weaken further.
No countries were given advance warning of a nuclear test,South Korea's intelligence service said, according to lawmakersbriefed by intelligence officials.
In previous such tests, Pyongyang had notified China, Russiaand the United States beforehand, they said.
U.S. PRESIDENTIAL CANDIDATES
While a fourth nuclear test had long been expected, theclaim that it was a hydrogen device, much more powerful than anatomic bomb, came as a surprise, as did the timing.
It made North Korea a topic on the U.S. presidential campaign with the first state nominating contests weeks away. The vote is in November.
Democratic front-runner Hillary Clinton condemned the test as a "provocative and dangerous act" that the United States should meet with sanctions and strengthened missile defenses.
"North Korea must have no doubt that we will take whatever steps are necessary to defend ourselves and our treaty allies, South Korea and Japan," she said in a statement.
Republican candidate Donald Trump said the onus was on China to solve what he called the North Korean "problem", and if it did not, the United States "should make trade very difficult forChina."
North Korea has long coveted diplomatic recognition fromWashington, but sees its nuclear deterrent as crucial toensuring the survival of its third-generation dictatorship.
While the Kim government boasts of its military might to project strength globally, it also plays up the need to defend itself from external threats as a way to maintain control domestically.
The North's state news agency said Pyongyang would act as a responsible nuclear state and vowed not to use its nuclear weapons unless its sovereignty was infringed.
Michael Madden, an expert on North Korea's secretive leadership, said, "With Iran being off the table, the North Koreans have placed themselves at the top of the foreign policy agenda as far as nation-states who present a threat to the U.S."
The device had a yield of about 6 kilotonnes, according tothe office of a South Korean lawmaker on the parliamentaryintelligence committee - roughly the same size as the North'slast test, which was equivalent to 6-7 kilotonnes of TNT.
"Given the scale, it is hard to believe this is a realhydrogen bomb," said Yang Uk, a senior research fellow at theKorea Defence and Security Forum.
Joe Cirincione, a nuclear expert who is president ofPloughshares Fund, a global security organisation, said NorthKorea may have mixed a hydrogen isotope in a normal atomicfission bomb.
"Because it is, in fact, hydrogen, they could claim it is ahydrogen bomb," he said. "But it is not a true fusion bombcapable of the massive multi-megaton yields these bombsproduce".
The USGS reported a 5.1 magnitude quake that South Korea said was 49 km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.
The test may mark an advance of North Korea'snuclear technology. The claim of miniaturising, which wouldallow the device to be adapted as a weapon and placed on amissile, would pose a new threat to the United States and itsregional allies, Japan and South Korea.
The North's previous miniaturisation claims have not beenindependently verified. Many experts also doubt whether theNorth possesses missile technology capable of reliablydelivering a warhead to the continental United States.
South Korea said it would take all possible measures,including possible U. N. sanctions, to ensure Pyongyangpaid the price.
"The government must now work closely with the internationalcommunity to ensure that North Korea pays the commensurate pricefor the latest nuclear test," President Park Geun-hye said in astatement. "We must respond decisively through measures such asstrong international sanctions."
Conventional atomic bombs split atoms from heavier elements such as uranium or plutonium. They occur in one stage. The process is called fission. Hydrogen bombs have a second stage after fission. This fusion stage releases much more energy.
(Additional reporting by Meeyoung Cho, Ju-min Park, James Pearson, Se; Young Lee, Christine Kim, Jee Heun Kahng, Jack Kim in Seoul,; Louis Charbonneau at the United Nations, Ayesha Rascoe and in; Washington, Megha Rajagopalan in Beijing and Takashi Umekawa in; Tokyo; Writing by Tony Munroe and Alistair Bell; Editing by Raju Gopalakrishnan, Mike Collett-White and Howard Goller)
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