While the North has boasted of mastering miniaturisation before, this is the first time Kim has directly claimed the breakthrough that experts see as a game-changing step towards a credible North Korean nuclear threat to the US mainland.
His 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.
Military tensions have surged in the region since the North carried out its fourth nuclear test in January and a long-range rocket launch last month.
The UN Security Council responded by imposing tough new sanctions last week, which Pyongyang has condemned and labelled as part of a US-led conspiracy to bring down Kim's regime by force.
"The nuclear warheads have been standardised to be fit for ballistic missiles by miniaturising them," Kim noted during a visit with nuclear technicians that was reported by state media on Wednesday.
- AFP