Stephen Bosworth arrived in Seoul late Sunday, this first stop in a tour of the main regional powers this week which comes amid a flurry of reports about the North's developing nuclear program. Analysts say the reports are most likely part of the North's time-honored strategy to gain leverage in negotiations during any aid-for-disarmament talks which have been stalled.

Bosworth will meet officials in Seoul Monday, before traveling to Tokyo and Beijing.

A foreign ministry official in Seoul said reports that the North was building a uranium enrichment facility would be raised during his visit, but could provide no further details.

U.S. nuclear scientist Siegfried Hecker of Stanford University said in a report released Saturday that he was shown a modern, small, industrial-scale uranium enrichment facility during a visit to the main Yongybon complex this month.

Hecker's report adds to worries the North is seeking a second way to obtain fissile material for atomic bombs.

The American, who has making his fourth such visit to the Yongbyon complex, said the uranium enrichment facility was recently completed and was said to be producing low-enriched uranium (LEU) destined for fuel for a new light-water reactor.

A South Korean government official said it was hard to verify the North's enriched uranium program, but information from the North indicated it may have worked on such a program since April last year, Yonhap news agency reported.

Hecker, along with two other visiting teams, have this month reported seeing construction of an experimental light-water reactor at Yongbyon. The sightings come around the same time as reports of activity at a nuclear test site.

North Korea, which walked out on talks to disable its atomic arms program last year, has said it wants to return to the negotiating table. Experts say this is a sign that sanctions are hurting its economy.

NO SURPRISE

Analysts say by showing off its nuclear hand, North Korea is seeking to boost its capability to win concessions in disarmament-for-aid, six-party talks.

"This is no surprise at all, though it is real concern," said Kim Tae-woo at the Korea Institute for Defense Analysis. "North Korea has said it wants to sit down for six-party talks, and this will absolutely increase its leverage."

It is difficult to verify any reports on North's nuclear programs because international nuclear inspectors were expelled from the country some 18 months ago.

The North's reported nuclear advances come nearly two months after Kim Jong-il initiated the transition of power to his youngest son, Kim Jong-un, and analysts say he wants to use nuclear muscle to boost his son's credentials with the powerful military.

Analysts say the younger Kim will continue with the North's nuclear program because it is the ultimate bargaining chip to win concessions in negotiations for aid.

The destitute North has said it wants to enrich uranium for peaceful purposes to use for its electricity needs.

North Korea, which conducted nuclear tests in 2006 and 2009, is believed to have enough fissile material from its separate, plutonium-based nuclear program to make between six and 12 atomic bombs.

Even though it has exploded nuclear devices, Pyongyang has not shown it has a working nuclear bomb.

But its nuclear program is seen as a direct threat to U.S. allies Japan and South Korea, and a proliferation risk given North Korea's long history of selling missile technology abroad.

"THEY CAN THINK WHAT THEY WANT"

Hecker said he saw "more than a thousand centrifuges all neatly aligned and plumbed." Gas centrifuges are used in uranium enrichment.

He said he was told the facility contained 2,000 centrifuges but his team was unable to verify that. The North Korean chief process engineer said all the components were manufactured domestically, Hecker wrote in his report.

"The control room was astonishingly modern," he wrote.

Hecker said he was unable to spend enough time there to establish whether the plant was designed to produce only low-enriched uranium needed to make fuel for a power plant or the highly enriched uranium needed for bombs.

"When I pointed out that the outside world will be concerned about their ability to convert the facility to make HEU (highly-enriched uranium), he stated that anyone can tell by looking at the monitors in the control room that the cascades are configured for LEU," he said.

"Besides, he said, they can think what they want."

Joel Wit, a visiting scholar at the U.S.-Korea Institute at Johns Hopkins University who also traveled with another group to Pyongyang this month, said his group was told by North Korean officials of plans to build an enrichment plant.

"They said they'd have fuel rods ready by the time the (light-water) reactor facility is ready," Wit, a former U.S. State Department official who dealt with North Korea, said in Beijing, where he flew to after Pyongyang.

The apparent sophistication of North Korea's enrichment program could ignite fresh debate over how to deal with its prickly leadership and whether to resume nuclear talks, stalled for two years.

North Korea has said it wants to return to talks but Seoul and Washington have dismissed its pledges to denuclearize as insincere.