Kim Jong-un says North Korea has miniaturised nuclear warheads

SEOUL — North Korean leader Kim Jong-un said his country has successfully miniaturised a thermo-nuclear warhead, as Pyongyang on Wednesday continued to talk up its nuclear strike capabilities amid rising military tensions on the Korean peninsula.

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.

Kim also stressed that the miniaturised warheads were "thermo-nuclear" devices, echoing the North's claim that a 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.

"This can be called a true nuclear deterrent," he was quoted as saying.

The North Korean ruling party's newspaper, Rodong Sinmun, carried a large front-page picture of Kim standing in front of what some experts said would appear to be a sized-down device.

"Obviously we only have the picture to go on, but it looks as you would expect for a compact nuclear warhead," said Jeffrey Lewis, director of the East Asia Nonproliferation Programme at the James Martin Centre for Nonproliferation Studies of the Middlebury Institute of International Studies (MIIS) in California.

Melissa Hanham, another expert on North Korea's WMD programme at MIIS, said Pyongyang's nuclear programme had been running long enough, with enough tests, to make it "distinctly possible" that effective miniaturisation had been achieved.

"I don't know that they could target that missile very well, or what it's range might be, but the claim cannot be dismissed as bluster," Hanham said.

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.

Military tensions have surged on the divided Korean peninsula since the North's 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 miniaturisation issue is key as, while North Korea is known to have a small stockpile of nuclear weapons, its ability to deliver them accurately to a chosen target on the tip of a ballistic missile has been a subject of heated debate.

There are numerous question marks over the North's weapons delivery systems, with many experts believing it is years from developing a working inter-continental ballistic missile that could strike the continental United States.

It is also unclear whether any miniaturised device the North has designed would be robust enough to survive the shock, vibration and temperature change associated with ballistic flight.

Most experts rule out the prospect of North Korea launching any sort of nuclear strike with a largely untested system, saying it would be tantamount to suicide given overwhelming US technical superiority.

"Kim's remarks should really be seen in the context of the cyclical, bellicose language the North uses on an annual basis, especially in the wake of the UN sanctions," Hanham said.

"His comments and the photos are making the message very explicit: 'We have a nuclear weapon and you have to respect us,'" she added.

South Korea had a similar take, with the Unification Ministry saying the North was essentially reacting to the imposition of the new UN sanctions.

But a ministry spokesman noted that Seoul believed Pyongyang had secured nuclear-related miniaturisation technology "to a certain degree".

North Korea's claim to have successfully tested an H-bomb in January was greeted with scepticism at the time as the estimated yield was seen as far too low for a full-fledged thermo-nuclear device.

However, weapons experts have suggested it may have been a "boosted" fission device, which makes more efficient use of nuclear material and can be made smaller without sacrificing yield.