SAN FRANCISCO -- U.S. officials were surprised at the high level of sophistication in North Korea's uranium enrichment facility, an American nuclear scientist said, when he gave them the first-hand account of what he saw in the communist nation.

   Siegfried Hecker, a Stanford University professor, visited North Korea's main nuclear complex in Yongbyon during a trip to the isolated nation in November. North Korean officials showed the U.S. scientist a uranium enrichment facility that could be used to make atomic bombs.

   North Korea has long been suspected of seeking a uranium-based nuclear weapons program, but it was the first time that the regime had publicly acknowledged the existence of the program and a facility for it, a move seen as an attempt to raise the stakes in its international nuclear standoff.

   "These are P-2 centrifuges whereas in Iran, because of international inspectors, they only have been able to make (less

sophisticated) P-1 centrifuges," Hecker said in an interview with Yonhap News Agency. "My analysis is, if what they (North Koreans) told me is correct, they have a very sophisticated second generation centrifuge at Yongbyon."

   The existence of such a facility appeared to be not surprising to U.S. officials because they have long held such suspicions, but the "surprise was that what I saw was so modern and sophisticated,"

Hecker said during the one-hour interview held Sunday.

   "There also is concern that if that's what they have, then they must have been doing this for a very long time," he said.

   Hecker also said that the North must have additional centrifuges elsewhere in the country.

   The enrichment plant of about 2,000 centrifuges could produce one nuclear bomb worth of weapons-grade uranium a year, but the timeline is not important because the North already has nuclear weapons, Hecker said.

  North Korea claims that the plant is to produce fuel for a light water nuclear reactor under construction for power generation. Few believe the claim by a regime that has pursued nuclear ambitions for decades and conducted two atomic test explosions in 2006 and 2009.

   Last week, U.S. President Barack Obama and Chinese President Hu Jintao held a summit in Washington and expressed "concern" about the North's uranium enrichment program.

   It was the first time that China, the North's last-remaining major ally, has taken a negative stance on the program. Up until then, Beijing had taken a vague stance, claiming the program should first be verified and that Pyongyang has the right to peaceful use of nuclear energy.

   The apparent change in China's position could be helpful for South Korea's push to take the issue to the U.N. Security Council.

Officials in Seoul said that the issue should first be dealt with at the Council before the six-party nuclear disarmament talks reopen.

   Hecker said the North's next step is expected to show the enrichment plant is working well.

   "I think from what they showed me and what they've proceeded, they may be ready to demonstrate that facility will be used for civilian nuclear fuel production," he said. "That will be the next step to show that it's working and it's making LEU (low enriched

uranium) for their LWR" (light water reactor).

   Hecker also raised safety concerns about the light water reactor.

   "North Korea is most likely doing this in isolation. What we've found over many decades is that it is important to do that through cooperation with countries that have nuclear safety expertise," he said.

   "One has to be concerned about accidents after the reactor begins operating. No nuclear explosion can happen from a reactor, but there could (be) radioactive contamination if there is accident."

(Yonhap News)