North Korea has developed the technology to fit a nuclear warhead on a missile, leader Kim Yong Un was quoted as saying by state media Wednesday.
The isolated regime has until now not been thought able to make its nuclear bombs small enough to be delivered by missile.
"The nuclear warheads have been standardized to be fit for ballistic missiles by miniaturizing them," Kim was cited as saying by state-run Korean Central News Agency. "This can be called true nuclear deterrent."
The report came two days after Pyongyang threatened a pre-emptive nuclear strike as US and South Korean forces started annual joint military drills in the region. North Korea says the manoeuvres are a rehearsal for invasion.
The government said last year it had made nuclear warheads small enough to fit on a ballistic missile, but experts in South Korea doubted this was accurate. Wednesday's was the first such statement directly from the North's leader.
"It is very gratifying to see the nuclear warheads with the structure of mixed charge adequate for prompt thermo-nuclear reaction," Kim also said.
Foreign observers have expressed doubt that North Korea has the ability to build a thermo-nuclear bomb, which would be more conventional than a standard nuclear bomb.
The UN Security Council last week imposed its strictest sanctions ever on Pyongyang, in response to the test of a nuclear device in January and a long-range rocket test last month.
The North Korean regime fired several short-range rockets off its eastern coast hours after the vote in New York Thursday.