North Korea has secretly and quickly built a new, highly sophisticated facility to enrich uranium, according to an American nuclear scientist, raising fears that the North is ramping up its atomic program despite international pressure.

The scientist, Siegfried Hecker, said in a report posted Saturday that he was taken during a recent trip to the North's main Yongbyon atomic complex to a facility with a small industrial-scale uranium enrichment facility. The facility had 2,000 recently completed centrifuges, he said, and the North told him it was producing low-enriched uranium meant for a new reactor.

Hecker, a former director of the US Los Alamos Nuclear Laboratory who is regularly given rare glimpses of the North's secretive nuclear program, acknowledged that it was not clear what North Korea stood to gain by showing him the formerly secret area.

The revelation could be designed to strengthen the North Korean government as it looks to transfer power from leader Kim Jong Il to a young, unproven son. As the North's economy suffers and Washington and others tighten sanctions, unveiling the centrifuges could also be an attempt by Pyongyang to force a resumption of stalled international nuclear disarmament-for-aid talks.

Whatever the reason, the new centrifuges provide a fresh set of worries for the Obama administration, which has shunned negotiations with the North following Pyongyang's nuclear and missile tests last year and in the wake of an international finding that a North Korean torpedo sank a South Korean warship in March, killing 46 sailors. The US State Department announced that the Obama administration's special envoy on North Korea planned to visit South Korea, Japan and China, starting Sunday.

Hecker wrote that his first glimpse of the new centrifuges was "stunning."

"Instead of seeing a few small cascades of centrifuges, which I believed to exist in North Korea, we saw a modern, clean centrifuge plant of more than a thousand centrifuges, all neatly aligned and plumbed below us," Hecker, a Stanford University professor, wrote.

Hecker described the control room as "astonishingly modern," writing that, unlike other North Korean facilities, it "would fit into any modern American processing facility."

The facilities appeared to be primarily for civilian nuclear power, not for North Korea's nuclear arsenal, Hecker said. He said he saw no evidence of continued plutonium production at Yongbyon. But, he said, the uranium enrichment 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 program. At low levels, uranium can be used in power reactors, but at higher levels it can be used in nuclear bombs. Hecker's findings were first reported in The New York Times.

US nuclear envoy Stephen Bosworth's trip to Asia for talks on North Korea comes as new satellite images show construction under way at North Korea's main atomic complex. That, combined with reports from Hecker and another American expert who recently traveled 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. 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 Bosworth was to arrive in Seoul on Sunday for a two-day trip aimed at discussing the North's nuclear weapons program. The US State Department said in a statement that Bosworth will then travel to Tokyo and Beijing.

A calculated disclosure

Yang Moo-jin, a professor at the Seoul-based University of North Korean Studies, said the North's uranium disclosure is meant to force the United States back into nuclear negotiations.

The disclosure, Yang said, also is aimed at a domestic audience during the succession process. "The North wants to muster loyalty among military generals by showing them the North will continue to bolster its nuclear deterrent and uphold its military-first policy," Yang said.

Light-water reactors are ostensibly for civilian energy purposes, but such a power plant would give the North a reason to enrich uranium. 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 weaponized 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 weapons. Hecker said the North Koreans emphasized the centrifuge facility was operating and while he couldn't verify that, "it was not inconsistent with what we saw."

Hecker wrote that many questions are still unanswered about North Korea's nuclear program, including whether the North is really pursuing nuclear electricity; whether it's abandoning plutonium production; how it got such sophisticated centrifuge technology; and why it's revealing the facilities now.

"One thing is certain," he wrote. "These revelations will cause a political firestorm."