In secret and with remarkable speed, North Korea has 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 yesterday that he was taken during a recent trip to the North’s main Yongbyon atomic complex to a small industrial- scale uranium enrichment facility.

It had 2,000 recently completed centrifuges, he said, and the North told him it was producing low-enriched uranium meant for a new reactor.

He described his first glimpse of the new centrifuges as “stunning.”

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 Washington and others tighten sanctions, unveiling the centrifuges could also be an attempt by Pyongyang to force a resumption of stalled inter- national nuclear disarmament-for-aid talks.

Whatever the reason, the new centrifuges provide a fresh set of worries for the Obama administration, which has shunned direct 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 today.

The North told Hecker it began construction on the centrifuges in April 2009 and finished only a few days before the scientist’s 12th Nov visit.

“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 saw no evidence of continued plutonium production at Yongbyon.

He said, the uranium enrichment facilities “could be readily converted to produce highly enriched uranium bomb fuel.”