The North has stepped up its bellicose rhetoric in response to the UN imposing some of its toughest sanctions.
In October 2014, the commander of US forces in South Korea, Gen Curtis Scaparrotti, told reporters that he believed the North had the capability to miniaturist a nuclear device.
Kim Jong-un has claimed North Korean scientists have developed nuclear warheads small enough to fit on ballistic missiles.
State media published images showing the North’s leader standing next to what it said was a miniaturised weapon.
The claim is impossible to verify from the images alone and experts have long cast doubt on such assertions.
In recent days, Pyongyang has threatened to launch an “indiscriminate” nuclear strike on the US and South Korea, as they began their largest ever round of annual military exercises.
The claim from Mr Kim was made as he inspected a nuclear facility on Wednesday.
“The nuclear warheads have been standardized to be fit for ballistic missiles by miniaturizing them,” state media agency KCNA quoted him as saying.
“This can be called true nuclear deterrent,” he added.
He also inspected nuclear warheads designed for thermo-nuclear reaction, the type used in a hydrogen bomb, KCNA said.
If the claim is true and North Korea can put nuclear warheads onto the tips of its ballistic weapons, it would represent a clear threat to the North’s neighbours and the US.
