North Korean leader Kim Jong Un visits the January 18 General Machinery Plant, where he specified tasks and methods for its modernisation, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on May 14, 2014. REUTERS/KCNA

By Jack Kim

SEOUL (Reuters) - North Korea, which this month threatened to carry out a fourth nuclear test, may be closer than previously thought to putting a nuclear warhead on a missile, some experts say, making a mockery of years of U.N. sanctions aimed at curbing such a programme.

North Korea has long boasted of making strides in acquiring a "nuclear deterrent", but there had been general scepticism that it could master the step of miniaturising a nuclear warhead to mount on a ballistic missile.

No one outside the inner circle of North Korea's nuclear programme likely knows what advances the country has made. But there has been a shift in thinking by some who study North Korea full time since it conducted a nuclear test in February last year and amid on-off indications it is preparing another.

The isolated and poverty-stricken state, which regularly threatens to destroy the United States and South Korea in a sea of flames, defends its nuclear programme as a "treasured sword" to counter what it sees as U.S.-led hostility.

And there was now "tremendous technological motivation" to conduct a nuclear test as it races to perfect the technology to miniaturise warheads, a South Korean nuclear expert said.

"The field deployment of a nuclear missile is imminent," said Kim Tae-woo, former head of South Korea's state-run Korea Institute for National Unification, who also served as head of research at the state-run Korea Institute for Defense Analyses.

Diplomatic sources told Reuters that China, North Korea's lone major ally, had used diplomatic channels to warn North Korea against a nuclear test, another possible sign that Pyongyang is considering such a move.

Experts say the delivery vehicle of choice for the North's first nuclear warhead would most likely be the mid-range Rodong missile, which has a design range of 1,300 km (800 miles).

"Given the number of years that North Korea has been working at it, my assessment is that they can mount a warhead on a Rodong," Mark Fitzpatrick, director of the non-proliferation at the International Institute for Strategic Studies, said.

"...Also, there is no doubt that Pakistan can mount a nuclear warhead on its version of the Rodong ...It is reasonable to assume that North Korea can too. How reliable the warhead would be is another question."

A South Korean government official involved in monitoring the North's nuclear capabilities said miniaturisation was "within sight".

"It is likely there has been progress, but on the question of whether they have actually achieved it, I'd have to say not yet," he said.

In March, the North fired two Rodong missiles which flew about 650 km (400 miles) before splashing into the sea off the east coast, well short of their full range.

Some experts interpreted the short flight as a test of a modified missile designed to carry a nuclear warhead by cutting the amount of fuel on board.

"A long-range missile test makes little sense for North Korea as a test to deliver a nuclear warhead," Kim said. "...if the North deploys a nuclear weapon, the strongest candidate to carry it will be the Rodong."

EARLY START

David Albright of the Institute for Science and International Security based in Washington, cited the low yields of the North's previous nuclear tests as consistent with the type of yield to be expected from a crude miniaturised warhead.

"North Korea is well aware of Pakistan's and Iran's work on miniaturising nuclear warheads for (their) missiles, which originally were copies of the Rodong missile," he said.

"North Korea would have likely made the same judgment as the two countries about the importance of starting early to develop a nuclear warhead for its missiles."

Ballistic missile launches are banned under U.N. Security Council resolutions. The council expanded sanctions after Pyongyang's February 2013 nuclear test, its third since 2006.

The sanctions target the missile and nuclear programmes and ban the export of luxury goods to the country, but they cannot seriously damage trade in a country that does little trade with the rest of the world.

For North Korea at present, what was likely more at stake was winning "the political poker game where risks and vague possibilities are seen as matter-of-fact situations", said Markus Schiller of Schmucker Technologie in Germany.

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