North Korea has said it has developed a more advanced nuclear weapon that has "great destructive power" and can be loaded onto an intercontinental ballistic missile (ICBM).
The North's official KCNA news agency on Sunday showed Kim Jong-un inspecting what it said was a hydrogen bomb that is to be loaded onto a new ICBM.
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 KCNA 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 the hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes, with its indigenously produced components allowing the country to build as many nuclear weapons as it wants.
Questions remain over whether Pyongyang has successfully miniaturised its weapons, and whether it has a working H-bomb, but KCNA quoted Kim as saying that "all components of the H-bomb were 100 percent domestically made", claims that have not been independently verified.
Al Jazeera's Wayne Hay, reporting from the South Korean capital Seoul, said it will be "almost impossible" for the US, South Korea and others to verify the North's claims.
Hay said, however, that "there will be a lot of concern about it regardless of whether it is real because it clearly shows that the North Koreans are serious about the message they are trying to get across and that they are serious about taking these provocations to another level".
The reports come less than a week after North Korea fired a ballistic missile over Japan, drawing strong international condemnation, including from the UN Security Council, which called the launch an "outrageous" threat and demanded that the country not launch any more missiles.
Last month, the Security Council unanimously imposed new sanctions on the North that could cut by a third the country's $3bn annual export revenue after it staged two long-range missile launches in July.
North Korea has been under UN sanctions since 2006 over its ballistic missile and nuclear programmes.
It has conducted launches at an unusually fast pace this year - at least 13 times, according to South Korean officials - and some analysts believe it could have viable long-range nuclear missiles before the end of US President Donald Trump's first term in early 2021.
A newly revealed US intelligence assessment indicates those missiles can carry nuclear warheads.
Last month, three short range North Korean ballistic missiles failed, which was considered a temporary blow to Pyongyang's rapid nuclear and missiles expansion, US military officials said.
The North has also threatened to fire missiles at the US territory of Guam.
The threat last month came after Trump warned that he would unleash "fire and fury" if North Korea continued its threats.
Source: Al Jazeera and news agencies