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In this Jan. 1, 2013 image made from video, North Korean leader Kim Jong-un speaks in Pyongyang, North Korea.KRT via AP Video/The Associated Press

North Korea's threat to conduct a "high-level nuclear test" and more long-range missile launches comes just days after the United Nations Security Council rebuked the regime and imposed new sanctions over a successful missile test conducted last month.

In a rather alarming section of the statement quoted by North Korean state media, the threats were "aimed at our arch-enemy, the United States." The statement continued: "Settling accounts with the U.S. needs to be done with force, not with words."

Behind the threats, however, is a nuclear weapons program that observers say has yet to master the important steps of creating a nuclear device small enough to serve as a missile warhead and has faced obstacles in creating an effective long range-missile. More testing might well aid the regime in its quest.

In December, North Korea launched a Unha-3 rocket that achieved orbit, a key step towards an intercontinental ballistic missile program that would put the most populated areas of the U.S. within target.

"Miniaturization is the big [challenge] – can you make a bomb small enough? And it's related to re-entry [from orbit] in the sense that you have to put shielding to soak up the heat or burn off the heat. So the smaller you can make the warhead the more space you've got for re-entry shielding," said Jeffrey Lewis, nuclear policy expert at the Monterey Institute of International Studies.

"Right now if you talk about North Korean delivery capability of a nuclear weapon, the main thing... is trying to make sure there weren't any North Korean trucks with nuclear weapons coming across the demilitarized zone into South Korea," said Scott A. Snyder, senior fellow for Korea studies at the Council on Foreign Relations and director of the program on U.S.-Korea Policy.

Another nuclear test would be the third since 2006 and the second during the Obama administration. The first two tests were conducted using plutonium. Observers believe North Korea has a limited supply of plutonium and are looking see whether the third test uses the process of uranium enrichment.

"In North Korea they actually have natural uranium reserves so they wouldn't have to import fuel in order to be able to process this for production and use in a weapon," said Mr. Snyder.

The North Korean incentive for pursuing a uranium enrichment program over plutonium is also tied with secrecy.

"The footprint of the [uranium enrichment] facility can be very small. It doesn't have any obvious external features. If you ever look at an enrichment facility, it looks like a warehouse. And there's no environmental signature associated with it. Reactors are very hot. It's super hard to hide an operating reactor – it's hot enough you can see the heat from space. But an enrichment facility is not like that at all," said Mr. Lewis.

In 2010, American nuclear weapons specialist Siegfried Hecker was shown a previously unknown North Korean uranium enrichment facility. "I was not surprised that Pyongyang finally admitted to having a uranium enrichment program; however, I was stunned by the size and sophistication of the 2,000 centrifuges in the cascade hall visible from the ultra-modern second-floor control room," said Mr. Hecker recounted in an article for the Bulletin of the Atomic Scientists.

Last summer, Mr. Hecker and Frank V. Pabian offered their evidence of preparations for a test tunnel to be used for a third nuclear test. Using plutonium as the key component of its nuclear weapons program was a "dead-end route" in large part because North Korea closed its only plutonium producing facility in 2007, said the authors. But, as they argued, a third nuclear test could use both plutonium and enriched uranium simultaneously to increase North Korea's technical knowledge.

"It is therefore conceivable that North Korea may conduct two tests simultaneously, using a double fishhook at the end of the tunnel, with one bomb fueled by HEU [highly enriched uranium] and a second by plutonium," wrote Mr. Hecker and Mr. Pabian.

"If Pyongyang has more HEU than we surmise, it may do more than one HEU test with different designs. Two detonations will yield much more technical information than one, and they will be no more damaging politically than if North Korea conducted a single test," they added.

Beyond the question of North Korea's nuclear and missile delivery capability is the question of what it would take for the North Koreans to feel satisfied with their program, explains Mr. Lewis, nuclear policy expert at the Monterey Institute of International Studies.

"The really interesting challenge for the North Koreans is that they test all kinds of things that don't work – how do you assess that as a weapons system? They may be happy with 50 per cent or 10 per cent reliability because it is a deterrent… We wouldn't be happy with that, but they might," he said.

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