SEOUL, South Korea — North Korea announced Wednesday that it successfully conducted a hydrogen bomb test, a claim that if true would mark a huge jump in Kim Jong Un's quest to improve its still-limited nuclear arsenal.
The announcement came after South Korea reported a seismic event resembling an earthquake 30 miles from the Punggye-ri site where the desperately poor and reclusive North has conducted nuclear tests in the past.
However, South Korean officials and some experts questioned whether the explosion was indeed a full-fledged test of a hydrogen device. Speaking on condition of anonymity, a senior South Korean military official told NBC News that "we presume that it was not a hydrogen bomb test."
North Korea's state-run KCNA news service reported that Kim "made the final decision on January 3 to go ahead with the hydrogen test and accordingly we have conducted hydrogen bomb test at 10 a.m. on January 6 with total success."
A television anchor announced that Pyongyang 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.
"Heaven and earth are shaking because of this historic event," she said. "It is our legal right as a sovereign nation to own hydrogen bomb for justice as we stand against the U.S., which is the culprit of invasion and who is looking for every opportunity to attack us with its vast pool of murderous nuclear weapons."
The North Korean newscaster also claimed that the "perfectly conducted experimental hydrogen bomb test did not create any negative impact on the ecology in the area."
The Associated Press reported that the announcement was celebrated on the streets of Pyongyang.
"If we didn't have powerful nuclear weapons, we would already have been turned into the slaves of the U.S.," university student Ri Sol Yong, 22, told the news agency.
North Korea conducted its last nuclear test in 2013. If Wednesday's claim is true, it would be the country's first involving a hydrogen bomb.
A hydrogen bomb, also called a thermonuclear weapon, produces a much more powerful blast than atomic weapons like the bombs dropped on Hiroshima and Nagasaki during World War II. H-bombs are much more difficult to make than atomic bombs.
In December, Kim claimed North Korea had a hydrogen bomb, but the White House and U.S. officials said there was no solid evidence it had successfully built such a device.
U.S. officials said it could take days to verify North Korea's latest claim.
"While we cannot confirm these claims at this time, we condemn any violation of U.N. Security Council resolutions and again call on North Korea to abide by its international obligations and commitments," White House National Security Council spokesperson Ned Price said late Tuesday. "We have consistently made clear that we will not accept it as a nuclear state."
Price added that the U.S. will continue to defend its allies in the region, including South Korea, and "will respond appropriately to any and all North Korean provocations."
South Korean President Park Geun-hye called the test "an act that threatens our lives and future." Japanese Prime Minister Shinzo Abe added: "We absolutely cannot allow this."
United Nations Secretary General Ban Ki-moon said the "deeply troubling" test violates "numerous" Security Council resolutions.
"This act is profoundly destabilizing for regional security and seriously undermines international non-proliferation efforts. I condemn it unequivocally," he said in a statement.
China, which is one of North Korea's few allies, also criticized the test. Foreign Ministry spokeswoman Hua Chunying said the government in Beijing "strongly opposes" it and urged Kim's regime to "stop any action which could worsen the situation."
South Korean lawmaker Lee Cheol Woo told reporters that the country's spy agency told him in a private briefing that Pyongyang may not have conducted a hydrogen bomb test given the relatively small size of the seismic wave reported.
He said the National Intelligence Service told him that an estimated explosive yield of six kilotons and a quake with a magnitude of 4.8 were detected Wednesday. Lee, a member of the parliamentary intelligence committee, said the agency told him that even a failed hydrogen bomb detonation typically yields tens of kilotons.
"Given the scale, it is hard to believe this is a real hydrogen bomb," Yang Uk, a senior research fellow at the Korea Defence and Security Forum, told 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."
Professor Jin Canrong, an international relations expert based at Renmin University in Beijing, also said he didn't believe a hydrogen bomb had been detonated.
"The explosive power of this test is only like that of their nuclear test in 2013 ... similar to the atomic bombs used by the U.S. in Japan during World War II," he told NBC News. "But they might have detonated a miniature atomic bomb."
Canrong said the test was likely aimed at spelling out to the North Korean public that "the young leader [Kim] is strong, that the country is strong."
Francois Heisbourg, a special adviser at the Paris-based Foundation for Strategic Research, also said he'd be "very surprised" if the test involved a true H-bomb.
"It could be what is called a boosted device — halfway between an ordinary Hiroshima or Nagasaki-type atomic weapon and an H-bomb," he added.
The nuclear test came two days ahead of what is believed to be Kim's birthday.
The Korean Peninsula remains technically at war, as a 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.
U.S. intelligence officials told NBC News in April 2014 that they believed the North had between a dozen and "a few dozen" missile-deliverable weapons.
The United Nations has imposed sanctions against North Korea, banning trade and financing activities relating to its nuclear weapons program in response to Pyongyang's previous nuclear tests.
The U.S. has also imposed its own sanctions against North Korea for years. In 2015, President Barack Obama announced new sanctions against the country following a cyberattack on Sony Pictures.
The hack has been seen as retaliation for the Sony film "The Interview," a comedy that depicted the fictional assassination of Kim. North Korea has denied it was involved in the cyberattack.