The U.S. is keeping close watch on Syria and North Korea, the Pentagon chief said Sunday, amid suspicions the Koreans are possibly cooperating with Syria on a nuclear facility.

"I think it would be a real problem," Defense Secretary Robert Gates said when asked how the Bush administration would view such an effort.

A senior U.S. nuclear official said Friday that North Koreans were in Syria and that Syria may have had contacts with "secret suppliers" to obtain nuclear equipment.

Andrew Semmel, acting deputy assistant secretary of state for nuclear nonproliferation policy, did not identify the suppliers, but said North Koreans were in Syria and that he could not exclude that the network run by the disgraced Pakistan nuclear scientist A.Q. Khan may have been involved.

Gates was asked in a broadcast interview whether Syria was involved in a covert nuclear program with North Korea's assistance.

"I'm not going to get into things that may involve intelligence matters, but all I will say is we are watching the North Koreans very carefully. We watch the Syrians very carefully," Gates said.

He added, "If such an activity were taking place, it would be a matter of great concern because the president has put down a very strong marker with the North Koreans about further proliferation efforts. And obviously, any effort by the Syrians to pursue weapons of mass destruction would be a concern for us."

A state-run newspaper in Syria said in an editorial Sunday that "the magnitude of these false accusations might be a prelude to a new aggression against Syria." Al-Thawra said suggestion of such nuclear cooperation was "a flagrant lie."

North Korea's minister to the country's U.N. mission in New York, Kim Myong Gil, has dismissed the Syria allegation as "groundless," South Korea's Yonhap news agency quoted Kim as saying.

This week, negotiators from six nations plan to meet in Beijing to discuss ways to disable North Korea's nuclear reactor.

North Korea agreed in a February accord to scrap its nuclear programs in return for political concessions and aid. The North has shut down its Yongbyon nuclear facility and negotiators are now discussing the next phase of the agreement: disclosing and disabling all nuclear facilities, which the North recently agreed to do by the end of the year.

Gates spoke on "FOX News Sunday."