North Korea, which this month threatened to carry out a fourth nuclear test, may be closer than previously thought to mounting a nuclear warhead on a missile, some experts say, making a mockery of years of U.N. sanctions aimed at curbing its efforts to obtain nuclear weapons.

North Korea has long boasted of making strides in acquiring a “nuclear deterrent,” but there has been general skepticism that it can master the step of miniaturizing a nuclear warhead to fit on a ballistic missile.

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

The isolated and poverty-stricken country, which regularly threatens to destroy the United States and South Korea in a sea of flames, defends its nuclear program as a “treasured sword” to counter what it sees as American-led hostility. And there is now “tremendous technological motivation” to conduct a nuclear test as the North races to perfect the technology to miniaturize 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.

Diplomatic sources told reporters that China, the North’s lone ally, had used diplomatic channels to warn Pyongyang against a nuclear test — another possible sign that it is considering such a move.

Experts say the delivery vehicle of choice for North Korea’s first nuclear warhead would most likely be the midrange Rodong missile, which has a range of 1,300 km.

“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 nonproliferation and disarmament 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 miniaturization was “within sight,” but added, “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.”

In March, the North fired two Rodong missiles, which flew about 650 km before splashing into waters off the country’s 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,” said Kim, who also served as head of research at the state-run Korea Institute for Defense Analyses. “If the North deploys a nuclear weapon, the strongest candidate to carry it will be the Rodong.”

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 miniaturized warhead.

“North Korea is well aware of Pakistan’s and Iran’s work on miniaturizing 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 programs and ban the export of luxury goods to the country, but they cannot seriously damage trade in a country cut off from the rest of the world.

For North Korea at present, what is likely more at stake is 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.

For a nuclear missile to reach its target with precision and undamaged from the stress of launch and re-entry, everything must work flawlessly and that could be achieved only through repeated testing, said Schiller, a missile technology expert.

A midrange Rodong would still require a flight into space and return to the atmosphere, bearing the full stress of the re-entry of peak loads of almost 20 times the force of gravity for a few seconds, he said.

“The big question is whether this warhead would still function after re-entry,” Schiller said. “My current guess is rather no than yes.”

But putting most of Japan within range of a nuclear-tipped ballistic missile will be sobering for Tokyo and its ally, Washington. “If you can take Tokyo hostage with nuclear weapons, you can do a lot of things,” said Narushige Michishita, a defense expert formerly involved in Japan’s security policy.

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