North Korea said Wednesday that it had carried out its first successful hydrogen bomb test after a tremor equivalent to a magnitude 5.1 earthquake rocked the reclusive communist state.
The U.S. Geological Survey registered the seismic activity at 10 a.m. in North Korea. The agency said the epicenter was about 235 miles northeast of the capital, Pyongyang, near the country's Punggye-ri nuclear test site.
An announcer for state-run television said the country had carried out the test of a "miniaturized" H-bomb. The country's official news agency said the test meant North Korea had "proudly joined the advanced ranks of nuclear weapons states … equipped with the most powerful nuclear deterrent."
The news agency said the test was intended to "firmly protect the sovereignty of the country" from the "ever-growing nuclear threat and blackmail by the U.S.-led hostile forces." It described the U.S. as a "gang of cruel robbers which has worked hard to bring even a nuclear disaster" to North Korea.
The White House was "monitoring and continuing to assess the situation in close coordination with our regional partners," said John Kirby, a spokesman for the U.S. State Department.
"We have consistently made clear that we will not accept [North Korea] as a nuclear state," he said. "We will continue to protect and defend our allies in the region."
The news prompted South Korea's Foreign Ministry to convene an emergency meeting, that nation's official Yonhap news agency reported. Japanese Prime Minister Shinzo Abe condemned the test as a threat to his country's security.
North Korea has previously conducted three nuclear tests at Punggye-ri, in 2006, 2009 and 2013. But those devices were believed to be plutonium-based.
A hydrogen bomb, which uses fusion to set off a chain reaction, is generally more powerful than a conventional nuclear bomb. But experts said North Korea's claim that the device that exploded Tuesday was a hydrogen bomb would need to be verified.
North Korean leader Kim Jong Un said in December that his country possessed an H-bomb, but the claim was met with skepticism by some specialists.
Joe Cirincione, president of the Ploughshares Fund, a global security foundation and author of "Nuclear Nightmares: Securing the World Before It Is Too Late," said Wednesday that he doubted North Korea had exploded a "real" hydrogen bomb. More likely, he said, Pyongyang detonated a "boosted" weapon with tritium added to increase the yield of a fission bomb.
North Korea has refused to give up its nuclear program despite international sanctions and warnings.
For years, the United States, China, Russia and Japan and South Korea sought to persuade Pyongyang to abandon the program through a series of multilateral talks, but those negotiations collapsed in 2009.
Unlike the 2013 test, there was no explicit warning from North Korea that it was preparing to conduct a test. But Tuesday, Pyongyang's official Korean Central News Agency said in a commentary that it had legitimate cause to develop a nuclear weapons program.
"We deserve to hold nuclear weapons and ceaselessly strengthen our byeongjin policy to counter nuclear threats by the U.S.," the commentary said. Byeongjin refers to the simultaneous pursuit of nuclear weapons and economic development.
In his new year's address Friday, Kim said celebrations last October marking the 70th anniversary of the Workers' Party "shook the world with the power greater than that of an explosion of an atomic bomb."
He also complained that the U.S. had "clung to its anachronistic policy" toward North Korea, "escalating the tension and egging its vassal forces on to stage a 'human rights' racket against the country."
"No plots and schemes of the enemy can break the indomitable will of our service personnel and people to firmly defend and add brilliance to our style of people-centered socialism, the base of their happy life," he said.
Times staff writer W.J. Hennigan in Washington and the Associated Press contributed to this report.