North Korea has secretly and quickly built a new facility to enrich uranium, according to a US nuclear scientist.
Siegfried Hecker said that he was taken during a recent trip to the North to a plant with a small industrial-scale uranium enrichment facility.
He said it had 2,000 recently completed centrifuges. The North told him it was producing low-enriched uranium meant for a new reactor.
Mr Hecker said his first look at the centrifuges was "stunning" and that the "uranium enrichment facility was ultramodern and clean".
He said the facilities appear to be primarily for civilian nuclear power.
Mr Hecker, former director of the US Los Alamos Nuclear Laboratory and a regular visitor to the North, said he saw no evidence of plutonium production.
But he said the facilities "could be readily converted to produce highly enriched uranium bomb fuel".
Uranium enrichment would give the North a second way to make atomic bombs, in addition to its known plutonium-based programme.
Mr Hecker's findings were first reported in The New York Times.
The US State Department today announced that the Obama administration's special envoy to North Korea planned to visit South Korea, Japan and China.
Stephen Bosworth's trip comes as new satellite images show construction under way at North Korea's main atomic complex. That, combined with reports from Mr Hecker and another American expert who recently travelled to Yongbyon, appear to show that Pyongyang is keeping its pledge to build a nuclear power reactor.
North Korea vowed in March to build a light-water reactor using its own nuclear fuel. Mr Hecker, and Jack Pritchard, a former US envoy for negotiations with North Korea, have said that construction has begun.
South Korea's Foreign Ministry said that Mr Bosworth was due to arrive in Seoul for a two-day trip aimed at discussing the North's nuclear weapons programme. The US State Department said in a statement that Bosworth will then travel to Tokyo and Beijing.
Light-water reactors are ostensibly for civilian energy purposes, but the power plant would give the North a reason to enrich uranium. At low levels, uranium can be used in power reactors, but at higher levels it can be used in nuclear bombs.
While light-water reactors are considered less prone to misuse than heavy-water reactors, once the process of uranium enrichment is mastered, it is relatively easy to enrich further to weapons-grade levels.
North Korea said last year it was in the final stage of enriching uranium, sparking worries that the country may add uranium-based weapons to enlarge its stockpile of atomic bombs made from plutonium. Experts say the North has yielded enough weaponised plutonium for at least a half dozen atomic bombs.
Enriched uranium would provide the North with an easier way to build nuclear bombs compared to reprocessing plutonium. Uranium also can be enriched in relatively inconspicuous factories that are better able to evade spy satellite detection, according to US and South Korean experts.
Uranium-based bombs may also work without requiring test explosions like the two carried out by North Korea in 2006 and 2009 for plutonium-based