North Korea has confirmed suspicions that it has restarted operations at its Yongbyon nuclear plant and threatened to use nuclear weapons against the US "at any time".
The North has made a number of "innovations" in its nuclear weapons programme "by steadily improving the levels of nuclear weapons . . . in quality and quantity", an official of the Atomic Energy Institute told state media.
Extensive upgrades had been carried out at the Yongbyon facility, including improvements to the uranium enrichment plant and the five-megawatt nuclear reactor.
The official said Pyongyang is "fully ready to deal with" the US and other countries that Pyongyang claims are hostile towards North Korea "with nuclear weapons at any time".
The Yongbyon plant had been shut in 2007 under an agreement with five countries, including the US and China, in exchange for diplomatic and economic incentives, but North Korea promised it would restart it two years ago at the height of tensions arising from the North's anger over joint military drills by US and South Korean forces.
North Korea's renewed belligerence comes just weeks after it reached an agreement with South Korea to defuse tensions on the border, after an exchange of artillery fire brought the two sides close to a broader clash.
Analysts believe Kim Jong-un, the North Korean leader, lost a significant amount of face after backing down at the height of the crisis and he is attempting to once again assert his position.
"This could have been expected," said Rah Jong-yil, a former head of South Korean intelligence. "They are using the same old trick of making a nuisance of themselves, which is the only thing they can realistically do in the present circumstances," he said.
"The North wants to make its presence felt in the international community, so they are defying South Korea and the United States, while they continue to cold-shoulder China," Mr Rah said, adding that Mr Kim's actions were more a sign of petulance.
"They do not have other ways of making themselves relevant," he said.
The resumption of operations at Yongbyon is, nevertheless, a cause for concern in other countries in the region.
The International Atomic Energy Agency warned recently that satellite images of Yongbyon indicated that North Korea had apparently restarted the nuclear reactor. The IAEA has been forced to rely on intelligence assessments of the North's nuclear capabilities after Pyongyang ordered all its inspectors to leave the country in 2009.
Analysis carried out by 38 North, the respected website operated by the US-Korea Institute at Johns Hopkins School of Advanced International Studies, said the "significant increase" in activity at the plant suggested that Pyongyang was attempting to produce new weapons-grade plutonium.
The graphite-moderated reactor is capable of producing fuel rods which, if reprocessed, could provide the regime with sufficient plutonium to manufacture one nuclear warhead a year. North Korea has spent decades trying to develop operational nuclear weapons.
It is thought to have a small arsenal of atomic bombs and an impressive array of short- and medium-range missiles. But it has yet to demonstrate that it can produce nuclear bombs small enough to place on a missile, or missiles that can reliably deliver their bombs to faraway targets.
Still, it has conducted three nuclear tests and a series of steadily improving long-range rocket launches, and some analysts see the announcements as foreshadowing another launch ahead of the anniversary celebration or a fourth nuclear test, which would push North Korea further along in its nuclear aims.
At various points in the decades-long standoff over its nuclear ambitions, North Korea has said it has shut down or restarted its atomic fuel production. In 2013, it said it would restart a plutonium reactor that had been shuttered under a 2007 disarmament agreement.
Satellite imagery earlier this year showed signs it still wasn't operating fully.
A uranium enrichment facility unveiled to a visiting American scientist in 2010 presumably gives North Korea a second way to make fissile material for bombs. (© Daily Telegraph London)