Seoul (AFP) - 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.

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.

- A 'true' deterrent -

"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.

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

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

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 Program at the James Martin Center for Nonproliferation Studies of the Middlebury Institute of International Studies (MIIS) in California.

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South Korea's defence ministry was sceptical, saying its own assessment was that North Korea had "not yet secured miniaturised nuclear warheads".

The miniaturisation issue is key because, 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.

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

- Question marks -

"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.

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 (ICBM) 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.

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.