North Korea has developed a hydrogen bomb which can be loaded into the country's new intercontinental ballistic missile, the official Korean Central News Agency claimed Sunday.
Questions remain over whether nuclear-armed Pyongyang has successfully miniaturised its weapons, and whether it has a working H-bomb, but KCNA said that leader Kim Jong-Un had inspected such a device at the Nuclear Weapons Institute.
It was a "thermonuclear weapon with super explosive power made by our own efforts and technology", KCNA cited Kim him as saying, and "all components of the H-bomb were 100 percent domestically made".
Pictures showed Kim in black suit examining a metal casing with two bulges.
North Korea triggered a new escalation of tensions in July, when it carried out two successful tests of an ICBM, the Hwasong-14, which apparently brought much of the US mainland within range.
It has since threatened to send a salvo of rockets towards the US territory of Guam, and last week fired a missile over Japan and into the Pacific, the first time time it has ever acknowledged doing so.
US President Donald Trump has warned Pyongyang that it faces a rain of "fire and fury", and that Washington's weapons are "locked and loaded".
After Pyongyang carried out its fourth nuclear test, in January 2016, it claimed that the device was a miniaturised H-bomb, which has the potential to be far more powerful than other nuclear devices.
But scientists said the six-kiloton yield achieved then was far too low for a thermonuclear device.
When it carried out its fifth test, in September 2016, it did not say it was a hydrogen bomb.
The North had "further upgraded its technical performance at a higher ultra-modern level on the basis of precious successes made in the first H-bomb test", KCNA said Sunday, adding that Kim "set forth tasks to be fulfilled in the research into nukes".
Actually mounting a warhead onto a missile would amount to a significant escalation on the North's part, as it would create a risk that it was preparing an attack.