While Pyongyang has boasted of mastering miniaturisation before, this is the first time Kim has so explicitly claimed a breakthrough that experts see as a game-changing step for the North's nuclear capabilities.

Kim also stressed that the warheads were 'thermo-nuclear' devices, echoing the North's claim that the fourth nuclear test it conducted in January was of a more powerful hydrogen bomb.

"The nuclear warheads have been standardised to be fit for ballistic missiles by miniaturising them," Kim noted during a visit with nuclear technicians, the North's official KCNA news agency said.

The North Korean ruling party's newspaper, Rodong Sinmun, carried a large front-page picture of Kim standing in front of a spherical metal object. The picture carried no caption and the paper did not explicitly claim this was part of the miniaturised warhead in question.

"Enough time has passed with North Korea's nuclear programme to make it distinctly possible it has miniaturised a device that can fit on a ballistic missile," said Melissa Hanham, an expert on North Korea's WMD programme at the Middlebury Institute of International Studies.

Kim's comments came a day after the North's powerful National Defence Commission threatened pre-emptive nuclear attacks on South Korea and the US mainland, as Seoul and Washington kicked off large-scale joint military exercises.

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