The statement came as an American scholar said North Korean officials had told him Pyongyang has weaponised 30.8 kg (68lb) of plutonium, enough for four or five bombs.

Selig Harrison, a North Korean expert at the Woodrow Wilson International Center for Scholars in Washington, said the North Koreans had told him the weapons cannot be inspected.

The officials were vague about what weaponisation meant, but it appeared most likely the plutonium would be fitted in missile warheads, Harrison said…

The tougher stance over nuclear weapons may be a consequence of a shift in power after the reclusive Kim suffered a stroke in summer, ­Harrison said. The ruling party has issued a stream of reports supporting its claim that the “Dear Leader” is well, but Harrison said Kim now appears to have given up handling day-to-day tasks.