Yugoslav security forces are infiltrating Kosovo to spy on NATO forces and provoke renewed violence, a senior U.N. official said today.

Military officers in the NATO-led peacekeeping force said their troops have been placed on a higher state of alert, particularly in the tense northern Kosovo city of Kosovska Mitrovica, where Serbs and ethnic Albanians clashed 10 days ago.

NATO officials also have warned U.N. officials, Western diplomats and ethnic Albanian leaders of increased security risks, causing some to add bodyguards, avoid nighttime travel and stay closer to colleagues, U.N. officials said.

"There is a convergence of alarm," a U.N. official said, adding that NATO may need to take the security threat more seriously. In particular, the official said, NATO should consider policing Kosovo's northern boundary with Serbia to help prevent what he described as an influx of too many "young people . . . who have no link to Kosovo."

Several NATO officials expressed much less concern, saying they have no evidence of specific plans by Yugoslav forces to create disturbances in Kosovo, which remains a province of Serbia, Yugoslavia's dominant republic. One official involved in monitoring Serbian activities in Kosovo described the intelligence reports as more rumor than fact.

The chief U.N. official in Kosovo, Bernard Kouchner, is nonetheless considering whether to restrict access to Kosovska Mitrovica for the first time since NATO's peacekeeping operation got underway in June, officials said. Under the draft rule, only those who could establish a prior connection to the city would be allowed to travel there.

"A lot of nonofficial people are coming," Kouchner said at a news conference today. "Some of the incidents have been organized . . . not only in Mitrovica."

The commander of NATO's Kosovo forces, British Lt. Gen. Mike Jackson, said, however, that "there is no evidence . . . that members or officials of state organizations are involved," despite the discovery several weeks ago that a Serb shot by Russian troops for harassing an ethnic Albanian had a Yugoslav Interior Ministry identity card.

Reuters reported from Belgrade:

Yugoslavia condemned the West's demobilization deal with the ethnic Albanian Kosovo Liberation Army, saying it would cause more Serbs to flee the province.

The KLA, under pressure from NATO and the United Nations, agreed Monday to terms to create a new civilian body in place of the separatist guerrilla force. The Justice Ministry in Belgrade said the accord sought to "legalize the action of terrorist bands and the criminal KLA," arguing that it was a violation of Yugoslav sovereignty in Kosovo.