North Korea likely has at least one secret nuclear facility, in addition to a plant at Yongbyon whose existence was reported last year, according to diplomats who have read a confidential United Nations report.

A UN panel of nuclear experts also said in the report that North Korea is receiving technical assistance from visiting foreign scientists, according to the diplomats who described the findings and asked not to be identified because the report hasn’t been made public. The panel reached its conclusions during a two-month investigation of the Yongbyon facility.

The Yongbyon plant’s existence was disclosed following the North Korean government-authorized visit of Siegfried Hecker, a professor at Stanford University in California.

The report, commissioned by the UN Security Council committee to implement sanctions against North Korea, says it’s likely that centrifuges and other equipment in the Yongbyon reactor were previously operating at one or more secret locations, according to the diplomats.

They said the report also cites evidence that foreign scientists visiting North Korea contributed their expertise to construction of the plant. No country of origin was identified.

The Security Council sanctions committee requested the report after a briefing by Hecker on Dec. 6. The panel of experts, appointed by the council last year to assess sanctions intended to block North Korea’s development of nuclear weapons and ballistic missiles, didn’t visit the facility. They relied on Hecker’s impressions and input from Japanese and South Korean government officials and other nuclear energy experts.

Security Council Resolutions

While saying he couldn’t comment on a confidential report, Mark Kornblau, a spokesman for the U.S. mission to the UN, said North Korea’s uranium enrichment program violates UN Security resolutions 1718 and 1874. Also, he said in an e-mailed statement, the program “is inconsistent with North Korea’s previous commitments, and compels tighter enforcement of existing sanctions.”

The facility at Yongbyon is intended to provide low- enriched uranium for a light-water reactor for the “peaceful purpose of meeting electricity demand,” the official Korean Central News Agency said on Nov. 30. It was the first time North Korea acknowledged the program, revealed by Hecker on Nov. 20 following his visit.

It’s unlikely that North Korea manufactured the equipment needed to build the facility, Hecker told the panel. It is more likely that North Korea imported some of the materials, assembled parts of the facility at other locations over many years, and transferred them to Yongbyon, he said.

The report makes 10 recommendations, including designating additional North Korean nuclear officials for a travel ban and foreign-asset freeze.