The United States and its allies today accused North Korea of being a danger to the region after it showed off its latest advances in uranium enrichment, but Washington said it was still open to talks.
The reported sighting of more than a thousand centrifuges at its main nuclear complex appears to confirm the impoverished North, which already has a plutonium-based bomb, is on the way to creating a second source of weapons-grade nuclear material.
It comes just as Pyongyang is pressing regional powers to resume talks on its atomic weapons programme - about the only real leverage it has with the outside world.
"It is the latest in a series of provocative moves by the DPRK ... it is a very difficult problem we have been struggling to deal with for 20 years," US envoy Stephen Bosworth told reporters in Seoul, referring to the North by its acronym.
"This is not a crisis, we are not surprised," said Mr Bosworth, who is on the first leg of a tour of east Asia.
"My crystal ball is foggy but I would never declare any process dead," he said when asked about the fate of regional six-party talks. "We have hope that we will be able to resuscitate (them)." The North's reported nuclear advances, including work on a light-water reactor, come nearly two months after Kim Jong-il started the transition of power to his youngest son, Kim Jong-un. Analysts say he wants to use nuclear muscle to boost his son's credentials with the military.
Washington is particularly worried by the threat of North Korea - whose ravaged economy has long relied heavily on arms exports - selling nuclear weapons material to other states. It has conducted two nuclear tests to date and is believed to have enough fissile material to make several nuclear warheads.
The latest flurry over Pyongyang's nuclear ambitions follows comments at the weekend by Siegfried Hecker of Stanford University that he had been shown more than a thousand centrifuges during a tour of the Yongbyon nuclear complex this month. North Korea said they were operational.
It is impossible to verify the North's claims, which it first announced last year. International inspectors were expelled from the country last year, but Washington has said since 2002 that Pyongyang had such a programme.
The North has said it wants to resume multilateral talks, but Washington and Seoul have said they will only consider a return to the negotiating table when Pyongyang shows it is sincere about denuclearisation.
By showing its nuclear hand, analysts say North Korea is seeking to gain leverage in any aid-for-disarmament negotiations in stalled six-way talks with regional powers China, Japan, South Korea, Russia and the United States.
South Korean government officials said the latest revelations, if true, posed "a very serious problem," adding they were in keeping with Pyongyang's typical pattern of behaviour.
Defence minister Kim Tae-young raised the possibility of redeploying US tactical nuclear weapons. He told lawmakers that such an option could be discussed next month at a newly created joint military committee to enhance deterrence against the North's nuclear programs.
The weapons are believed to have been removed from the South in the 1990s when Washington vowed protection under its nuclear umbrella.
Japan said it could not accept North Korea's nuclear advances, citing its own security worries as well as concerns for regional peace and stability.
US defence secretary Robert Gates dismissed the notion the enrichment program might be for energy production, saying North Korea had a nuclear arms programme for some time and probably had a number of nuclear devices.