Despite lingering differences in their positions, Russian and Western envoys presented a plan to end the Kosovo conflict to Yugoslav President Slobodan Milosevic today, and the Serbian parliament plans an emergency session to take up the proposal Thursday while talks continue.

Russian envoy Viktor Chernomyrdin and Finnish President Martti Ahtisaari, who is representing the European Union, flew to Belgrade following a late-night meeting outside Bonn with Deputy Secretary of State Strobe Talbott that almost scuttled the diplomatic mission.

Details of the plan were not known, but the trip was delayed for several hours after Chernomyrdin objected to last-minute clarifications put forward by Talbott that sought to underscore the West's insistence that any peacekeeping force in Kosovo must come under NATO command. Officials said Talbott also insisted that a suspension of NATO's bombing campaign must be linked to a sequenced withdrawal of Yugoslav and Serbian forces from Kosovo, a province of Serbia, Yugoslavia's dominant republic.

While not all issues were resolved, Ahtisaari said he was persuaded that enough progress was made in narrowing differences with Russia to justify his mission. His visit marked the West's first official contact with the Yugoslav leadership since NATO airstrikes began March 24.

"This is a historic day for Yugoslavia," Ahtisaari said before the talks began. "Mr. Chernomyrdin and I are coming with a peace plan, and we hope after these talks we can proceed with peace developments in the region."

Shortly after the discussions broke up this evening, NATO provided a dramatic illustration of its intention to continue its bombing campaign until Milosevic agrees to a peace settlement. At 10 p.m., air raid sirens sounded across Belgrade, and 15 minutes later lights throughout the city went out. NATO warplanes have repeatedly attacked Serbia's electrical grid in recent weeks, causing widespread power outages.

Today's meeting was the first in which peace talks reached a stage of intense give-and-take, and the increased momentum seemed reflected by the planned special session of the Serbian parliament. The last time the legislature met was in March, when it rejected a Western-drafted peace plan following multi-party negotiations outside Paris. NATO airstrikes against Yugoslavia began the following day.

U.S. officials said they do not expect the Chernomyrdin-Ahtisaari visit to produce an immediate agreement. Chernomyrdin too cautioned: "This is just the start. We have basic elements for a political settlement. If we agree on that, then we will come back, and the negotiations will be held in completely different circumstances and in a working atmosphere."

Secretary of State Madeleine K. Albright and other U.S. officials said they were encouraged, however, that Chernomyrdin had moved close enough to NATO's bottom-line position so that he and Ahtisaari could undertake their mission.

"We believe that this is a significant step forward," Albright said at a news conference in Washington with Macedonian Prime Minister Ljubco Georgievski. "Of course, now the big question that remains to be answered is how Milosevic will react." She added that the mission of Chernomyrdin and Ahtisaari is "not a negotiation" but an opportunity to explain to Milosevic exactly what the Belgrade government must do before NATO agrees to halt the airstrikes.

NATO's conditions include the withdrawal of all Yugoslav army and Serbian police and paramilitary forces from Kosovo and the deployment of an international "security presence" there with allied soldiers at its core. NATO also is demanding repatriation of all Kosovo refugees and an agreement on a political process geared toward establishing substantial political autonomy for the ethnic Albanian majority in Kosovo.

Chernomyrdin had carried specific Yugoslav proposals to Bonn that had been worked out last Friday during marathon meetings here with Milosevic. Among the proposals were offers to bridge disputes between NATO and Belgrade over the makeup of the Kosovo peacekeeping force and whether any Yugoslav forces may remain in the province.

The Russians -- and the Serb-led Yugoslav government in Belgrade -- had been demanding that as many as 24,000 troops and policemen be allowed to remain in Kosovo, according to Western sources. According to these sources, the Russians had argued that there are 8,000 Serbian Orthodox religious sites in Kosovo and that each had to be guarded by three soldiers.

At the talks Tuesday night in Germany, however, Chernomyrdin agreed to settle on a figure of 800 to 1,000 troops, which is much closer to the U.S. position that a very small, carefully circumscribed force of 200 to 300 should be allowed to return to Kosovo following a full withdrawal, the officials said.

The Belgrade government, meanwhile, listed only three NATO countries -- Norway, Portugal and Greece -- that it said it would permit to send troops to Kosovo as part of a peacekeeping force. In the view of officials here, troops from countries engaged in airstrikes against Yugoslavia -- such as the United States, Britain and France -- cannot be part of the force. The United States and NATO have rejected any such condition, and even on that point Belgrade appeared today to be flexible. One senior official here declined to rule out a role for Americans.

Foreign Ministry adviser Milisav Pajic said the government might agree to allow other NATO countries to take part in the peacekeeping force if a U.N. mandate for the mission stipulates that it must not take on the role of an aggressive "peace making" force with powers to impose rules on the Yugoslav army and police.

"It is very much a question of what the final political document looks like," Pajic said. "There must be a limited mandate for the peacekeepers. They must respect the sovereignty and territorial integrity of Yugoslavia." He said NATO proposals to bring more than 50,000 peacekeepers to Kosovo to protect hundreds of thousands of returning ethnic Albanian refugees is "too much."

Pajic expressed a sense of urgency because of the possible hardship for both ethnic Albanians and Serbs in the approaching winter. The homes of Kosovo Albanians, damaged during mass expulsions by government security forces, will have to be repaired, while NATO airstrikes have damaged the centralized heating systems of Serbian cities. "Time is running out; an agreement should be reached this month," he said.

After what he described as a "very difficult negotiating process" with Talbott and Ahtisaari, Chernomyrdin indicated that key differences over two important issues were left to be resolved later, something that NATO officials said could pose serious problems to the implementation of any cease-fire and eventual peace settlement.

Chernomyrdin said it is of critical importance that Yugoslavia and NATO reach an early understanding covering the withdrawal of Yugoslav forces from Kosovo and the timing of the deployment of international peacekeepers. He said that only then could an enduring cease-fire be assured, to be followed by a U.N. Security Council resolution encompassing the peace plan.

He also said there should be two separate peacekeeping presences in Kosovo -- a NATO force and a Russian force -- that would operate under separate commands. U.S. and NATO officials immediately contested this view. Talbott said the the separation of peacekeeping forces would lead to the partition of Kosovo. "There needs to be unity of command for this to be really effective, in order for it not to slip down a slippery slope into partition, ineffectiveness or worse," Talbott said.

British Foreign Secretary Robin Cook insisted that NATO must be allowed to deploy its forces in every sector of the province and must operate under a unified command supported by NATO headquarters. "We are not willing to enter into a partition of Kosovo, either by agreement or by the back door," Cook said.

The issue may not be as unresolvable as presented by NATO: In neighboring Bosnia, Russian peacekeepers operate alongside NATO troops but are not under NATO command.

Correspondent William Drozdiak in Brussels and staff writer Thomas W. Lippman in Washington contributed to this report.