Washington greeted the news that Pyongyang had tested a hydrogen bomb with a mixture of condemnation and scepticism yesterday, as United States experts said they were not yet able to verify the veracity of the claim.
White House National Security Council spokesman Ned Price said in a statement that it was aware of seismic activity near a known North Korean nuclear test site, and was "continuing to assess the situation in close coordination with our regional partners".
"While we cannot confirm these claims at this time, we condemn any violation of UN Security Council resolutions and again call on North Korea to abide by its international obligations and commitments," he added.
The US stressed it would not accept North Korea as a nuclear state and pledged to defend its allies South Korea and Japan. "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."
Much of the scepticism about the hydrogen bomb test centres on the fact that Pyongyang had, until 2013, conducted spotty atomic bomb tests. A hydrogen bomb is a much more difficult weapon to get right.
Dr Jeffrey Lewis, director of the East Asia Non-proliferation Programme at the James Martin Centre for Non-proliferation Studies, wrote an analysis last month evaluating Pyongyang's thermonuclear claims and concluded it was probably a stretch too far for the regime.
"Thermonuclear weapons are tricky; making one work requires a bit of test experience," he said, adding that it was more plausible that the North was experimenting with boosting its atomic bombs.
If Pyongyang's claim turns out to be true, it would present the US with a new nuclear headache just months after completing negotiations on a deal with Iran.
The presence of a North Korean thermonuclear weapon would mean a significant boost in the country's nuclear capability and likely trigger a new round of sanctions from the United Nations.
Some US lawmakers called for action against North Korea.
Senator Tom Cotton said: "In the near term, the US must lead with a firm response, including an enhancement of sanctions... and for the long-term security of the region... the US must reinvest in missile defence and our military presence in the Pacific."