TOKYO — 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.
"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.