North Korea's leader Kim Jong Un has inspected the loading of a hydrogen bomb into a new intercontinental ballistic missile (ICBM), the country's state media has said.
There will be some scepticism about the claim from experts about Pyongyang's assertion that it has mastered hydrogen technology.
But Sunday's statement by the state run Korean Central News Agency will raise already high worries on the Korean Peninsula and in Washington that the North is closer to its goal of an arsenal of viable nuclear ICBMs that can reach the US mainland.
KCNA said on Sunday that Mr Kim inspected the loading of an "H-bomb into the ICBM" during a visit to the North's nuclear weapons institute.
The North conducted two nuclear tests last year alone. The first involved what it claims to have been a hydrogen bomb and the second was its most powerful ever.
Experts and outside governments are sceptical of the hydrogen claim, but it is almost impossible to independently confirm North Korean statements about its highly secret weapons programme.
The key question is how far North Korea has got in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles.
South Korea's main spy agency has previously asserted that it does not think Pyongyang currently has the ability to develop miniaturised nuclear weapons that can be mounted on ballistic missiles.
Some experts think the North may have mastered this technology.
North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.
The White House says President Donald Trump spoke to Prime Minister Shinzo Abe of Japan to discuss "ongoing efforts to maximise pressure on North Korea".
It did not say whether the conversation came before or after the North's claim about the loading of a hydrogen bomb into a new ICBM.
According to the White House, the two leaders reaffirmed the importance of close cooperation between the US, Japan and South Korea in the face of the growing threat from North Korea.
Mr Trump also noted that he looks forward to continued trilateral co-ordination on the margins of the United Nations General Assembly.