Russia's attempt to mediate an end to the Kosovo conflict appears to be on the brink of failure because Moscow shows no inclination to endorse NATO's core demands and persuade Yugoslav President Slobodan Milosevic to accept them, senior U.S. and NATO officials said today.

As Russia's Balkans envoy, Viktor Chernomyrdin, met in Belgrade with Milosevic, U.S. and NATO sources said the Russian government was bracing for a breakdown in its diplomatic effort and preparing for a new level of tension in its relations with the West.

Chernomyrdin complained that Milosevic's indictment on war crimes charges by the U.N. tribunal in The Hague Thursday had damaged his peace mission and that it seemed unlikely to succeed until NATO halts its bombing campaign against Yugoslavia.

Chernomyrdin flew to Belgrade after holding marathon talks in Moscow with Deputy Secretary of State Strobe Talbott and Finnish President Martti Ahtisaari, the European Union envoy. Ahtisaari was supposed to accompany Chernomyrdin to Belgrade but refused to do so, indicating he was dissatisfied with the gaps remaining between the positions of Russia and NATO.

Chernomyrdin said on departure from Belgrade, returning to Moscow after 10 hours of talks with Milosevic, that he and Milosevic "worked out in detail our actions at the forthcoming most difficult talks" the Russian envoy will hold with Talbott and Ahtisaari. Talbott and Ahtisaari are expected to return to Moscow, possibly early next week, to meet again with Chernomyrdin.

"I hope that next time we will come to Belgrade together with Martti Ahtisaari. I am 95-97 percent certain of that," Chernomyrdin said, according to the Russian Tass news agency. At the same time, Chernomyrdin said with regret that not everything depended on him. "I don't feel very well, I am not satisfied with the fact that I have been to Belgrade for an umpteenth time but bombings are continuing."

NATO said its planes took advantage of good weather to strike Serbian forces, four airfields, six ammunition storage sites and oil storage sites and radio communications stations. Belgrade was plunged into darkness at dusk Thursday, and much of the city was without power today. Serbia's power company said damage was substantial, and electricity may not be restored completely for days to come.

[Early Saturday, the capital remained in darkness, as did other communities in northern Serbia including Yugoslavia's second biggest city, Novi Sad, and Pancevo, after NATO jets bombed two important power stations, residents and media reports said. The official Yugoslav news agency Tanjug said at least three missiles struck a Novi Sad television studio. The agency said the "almost completely destroyed" building had been hit for the fourth time.]

Talbott came to NATO headquarters here in Brussels to brief ambassadors from the 19 allied nations on the status of his talks with Chernomyrdin. He said there were still substantial differences over whether NATO would lead the peacekeeping force and whether all Serbian forces must withdraw from Kosovo, the Serbian province that is at the center of the conflict. Serbia is Yugoslavia's dominant republic.

Talbott insisted that Chernomyrdin was not authorized to negotiate on behalf of NATO or to broker a deal. "We are not talking to Milosevic except in one language, and that is bombing," Talbott said. "The key question is what the Yugoslav leadership is prepared to accept in moving from military conflict to a diplomatic and political phase."

He insisted NATO would not budge from key demands requiring the Serb-led government in Belgrade to withdraw all its military and police forces from Kosovo, accept an international security force under NATO command, allow the safe return of all ethnic Albanian refugees, and endorse a political process that would restore self-government to the Kosovo Albanians.

The United States and its allies contend that all Serbian forces must leave Kosovo to reassure the hundreds of thousands of ethnic Albanian refugees who have fled the separatist province. At a later date, a small number of troops would be allowed to return, possibly to conduct border patrols and to guard Serbian monasteries and other religious sites. NATO agrees that Kosovo should remain part of the Serbian republic and not become a separate state, as secessionist ethnic Albanian guerrillas are demanding.

Russia, however, says a significant number of Serbian forces should be allowed to remain permanently in Kosovo to avoid creating a security vacuum and allowing the Kosovo Liberation Army to gain the upper hand when a cease-fire occurs.

In his draft peace proposal, Chernomyrdin has suggested that a mix of NATO peacekeeping forces and Serbian troops could supervise a cease-fire. The Russians also want a neutral nation, such as Finland, to command the force under U.N. auspices.

Talbott said he made clear to Chernomyrdin that NATO would insist on the temporary removal of all Serbian forces and that a peacekeeping force must have NATO "at its core," which senior U.S. officials define as a force under the unified command of a NATO officer serving at an allied headquarters.

While the Clinton administration and top NATO authorities have urged Russia and other countries outside the alliance to contribute troops to such a force, they have ruled out any command constraints on NATO's role and insisted that allied soldiers must be present in every sector of the province to prevent any partition. They have already dismissed suggestions that Russian and Ukrainian peacekeepers, for example, could be placed in charge of a Serb-controlled sector in northern Kosovo while NATO forces patrolled the south.

Senior NATO officials said they were deeply skeptical about Chernomyrdin's mission, particularly because Milosevic's indictment on crimes against humanity is expected to reinforce the Yugoslav leader's intransigence.

NATO officials are also troubled by the political disarray in the Russian government and the clear rivalry between Chernomyrdin and Foreign Minister Igor Ivanov. Alliance sources describe Ivanov as "vehemently anti-NATO" and determined to sabotage any deal that would vindicate NATO's bombing campaign.

Ivanov met in Stockholm this week with U.N. Secretary General Kofi Annan and the United Nations' new Balkans envoy, former Swedish prime minister Carl Bildt. NATO officials said if Chernomyrdin fails on his current mission -- his fourth trip to Belgrade -- Ivanov may try to maneuver Annan and Bildt to create a new U.N. mediating channel. Senior Clinton administration officials don't trust Bildt, who frequently criticized U.S. policies when he was an international peace envoy in Bosnia. He also has questioned the wisdom of NATO airstrikes and warned that the bombing campaign would only destabilize the region further.

Correspondent David Hoffman in Moscow contributed to this report.

CAPTION: Yugoslav President Slobodan Milosevic, left, welcomes Viktor Chernomyrdin on the Russian envoy's fourth trip to Belgrade as part of his mediation effort.