North Korea has allowed a visiting U.S. nuclear scientist to see a large new uranium enrichment facility which raises new questions about Pyongyang's nuclear intentions, the New York Times reported over the weekend.

Stanford professor Siegfried S. Hecker, former director of the Los Alamos National Laboratory, told the paper about his visit to the center which apparently has been quickly built and which poses a new challenge to keep the North Koreans' nuclear activities in check.

Hecker's claims of seeing hundreds of centrifuges confirms U.S. concerns about that country's uranium enrichment program, the top U.S. military officer said on Sunday.

"From my perspective, it's North Korea continuing on a path which is destabilizing for the region," Admiral Mike Mullen, chairman of the U.S. military's Joint Chiefs of Staff, told CNN television's "State of the Union" program.

"It confirms or validates the concern we've had for years about their enriching uranium - which they deny routinely."

Hecker told the New York Times that he was stunned by the new facility's sophistication. He saw "hundreds and hundreds" of newly-installed centrifuges being operated from "an ultra-modern control room," the newspaper report said.

He told the paper that the North Koreans claimed they now have 2,000 centrifuges installed and in operation.

The report said US officials knew of no such plant in existence in April 2009 when U.S. and international inspectors were expelled by North Korea.

The speed with which the new facility was built strongly indicated that North Korea had foreign assistance and had managed to evade UN Security Council sanctions imposed on Pyongyang for refusing to submit to international controls of its nuclear program.

Hecker's visit to the facility, which had previously housed an old nuclear fuel production plant, took place November 12. He privately informed President Barack Obama of the visit a few days ago, the New York Times report said.

The administration has since then begun briefing allies and lawmakers about the development, amid US efforts to persuade China to exert more pressure on North Korea in the standoff over Pyongyang's nuclear activities.