European Union foreign ministers decided today to send their official envoy to Yugoslavia this week after meeting in urgent session to consider whether Belgrade may be willing to accept Western terms for a halt to the NATO bombing and a settlement of the Kosovo crisis.

Diplomats said the envoy, Finnish President Martti Ahtisaari, is scheduled to meet with Russia's special Balkans envoy Viktor Chernomyrdin and Deputy Secretary of State Strobe Talbott in Bonn on Tuesday. Ahtisaari would then travel to Belgrade with Chernomyrdin on Wednesday to meet with Yugoslav President Slobodan Milosevic and return to Germany on Thursday to discuss his trip with European leaders, who will be holding a summit conference in Cologne.

Ahtisaari's mission would be the first official contact between Milosevic and the Western powers since the NATO bombing began March 24.

The latest diplomatic initiative to end the Kosovo conflict came on a day when NATO bombs and missiles reportedly killed 27 Yugoslav civilians. The deadliest mishap was said to have occurred in the town of Surdulica, where two missiles reportedly hit a medical facility and a senior citizens' home, killing 27 people [Story on Page A12].

Yugoslavia's official news agency, Tanjug, reported that Milosevic, after meeting today with top advisers, is prepared to accept the principles for a diplomatic settlement endorsed by the Group of Seven industrial democracies and Russia. The eight powers have called for an end to all violence and government repression of ethnic Albanians in Kosovo; the withdrawal of Yugoslav troops and Serbian police units; the introduction of an international security presence; the safe return of all refugees to their homes and the establishment of an interim administration for Kosovo, a province of Serbia, Yugoslavia's dominant republic.

A declaration by the 15 European foreign ministers said Ahtisaari would attempt to get Belgrade "to translate its reported statements into a firm, unambiguous and verifiable commitment" to abide by the principles set forth by the G-7 and Russia.

But NATO's conditions for halting the bombing and resolving the crisis are more stringent than those of the eight powers, requiring all Yugoslav troops and Serbian police forces to evacuate Kosovo -- at least temporarily -- to allow the return of the more than 800,000 refugees that security forces drove from the province. It also requires that the international peacekeeping contingent to be deployed there have NATO troops at its core.

Allied military commanders say that serious plans for inserting such a peacekeeping force must be considered by the end of June if the refugees are to be escorted home before the first snow of October. According to that timetable, efforts at a diplomatic solution must be completed -- successful or not -- by the time leaders of the G-7 and Russia meet in Cologne June 18-20. Senior alliance officials say that failure to find a diplomatic solution by then will force the conflict into a new phase and require winterized shelters for Kosovo refugees in neighboring nations.

While expressing hope that diplomacy could still succeed, NATO officials said allied warplanes pressed ahead with a bombing campaign that was in its 69th day today. Allied planes flew nearly 800 sorties and bombed a wide array of Yugoslav army targets in Kosovo -- including 12 tanks, six armored personnel carriers, seven artillery batteries and two airfields.

Speaking to reporters in Moscow, Chernomyrdin reiterated that halting the NATO airstrikes is a major condition for any resolution of the Kosovo crisis. "The West must understand what might happen if this concrete decision is not made" at the Bonn talks, he said.

"The week should bring a certain decision . . . if everything proceeds according to plan. A framework for a positive solution for a Yugoslav settlement has been worked out," he said, adding that the "mechanisms and the principles for putting it into practice" are the major obstacles.

But U.S. and NATO officials expressed skepticism about Milosevic's intentions. They said the Yugoslav leader has indicated for the past two weeks that he is ready to accept the eight-power principles, but he has shown no willingness thus far to accept NATO's tougher demands.

Goran Matic, a Yugoslav cabinet minister without portfolio and close associate of Milosevic, said in an interview in Belgrade today that the eight-power declaration "is a chance for a political way out of this crisis. To go back to NATO's five points is to return the story to the very beginning." Matic said that there has been no decision about the size of the Yugoslav security force to be left in Kosovo and that Yugoslavia has agreed only to the use of NATO troops from Portugal and Greece in any Kosovo peacekeeping operation.

"We certainly welcome any positive development," said White House spokesman Mike Hammer. "But it's not clear, at this point, that all the terms have been accepted."

After completing his fourth trip to Belgrade last week, Chernomyrdin expressed satisfaction with the results of his latest mediation mission. Russian news reports said that during nine hours of talks with Milosevic, Chernomyrdin put forward a detailed peace proposal that met with the Yugoslav leader's approval.

According to the Russian Tass news agency, the plan calls for a U.N. force under the command of a neutral country to supervise the withdrawal of some Yugoslav forces from Kosovo and the return of ethnic Albanian refugees, but soldiers from NATO countries participating in the bombing campaign would be barred from entering Kosovo.

But senior U.S. officials said that any plan that prevents soldiers from the United States, Britain and France from enforcing the peace accord in every sector of Kosovo would fail to instill enough confidence in the refugees to overcome their fears of the Belgrade government and return to their homes.

Speaking at Arlington National Cemetery today, President Clinton reiterated his position that Americans must play a role in any peacekeeping force. But he also said that Europeans will contribute most of the troops and finance most of the rebuilding of Yugoslavia after peace is achieved.

"In this military campaign, the United States has borne a large share of the burden, as we must, because we have a greater capacity to bear that burden," the president said. "But . . . we have been strongly supported by our European allies [and] when the peacekeeping force goes in there, the overwhelming majority of people will be Europeans, and that when the reconstruction begins, the overwhelming amount of investment will be European."

In addition to disagreements about the composition of a peacekeeping force, another sticking point may be Russia's persistent defense of Belgrade's right to exercise sovereignty over Kosovo by keeping up to 11,500 troops there. That level was agreed to by Milosevic last October during preconflict negotiations but was abrogated within months when more than 40,000 Yugoslav troops and Serbian police operating in the province launched an all-out campaign to expel the bulk of Kosovo's majority ethnic Albanian population.

The NATO allies insist that all government forces must be withdrawn from Kosovo to allow the entry of a peacekeeping force and the return of the refugees. At a later date, a token force of Yugoslav troops would be allowed to reenter the province to maintain border patrols and guard Serbian Orthodox monasteries and other religious sites.

Chernomyrdin told reporters today after meeting here with the U.N. secretary general's envoy, Slovak Foreign Minister Eduard Kukan: "We want to achieve the most important result -- stop the military actions and give things over to [the United Nations] so that they could take over and go on working along the main direction."

The Tanjug report said that early approval of a U.N. Security Council resolution embracing the eight-power principles "should enable the transfer of the resolution of the crisis from the military to the political sphere." The statement was welcomed by Russia's new prime minister, Sergei Stepashin, who said "real chances are appearing for breaking the deadlock on Yugoslavia."

While eager to keep Russia involved in the peace process, U.S. and NATO officials have expressed doubts about whether Chernomyrdin is conveying the allied conditions to Milosevic fully. They have been urging Ahtisaari to become directly involved in the mediation effort because they believe he will explain to Milosevic that NATO will not compromise on its bottom-line demands.

"We have still not heard, from Milosevic or the Russians, that the Yugoslav authorities are willing to allow NATO soldiers under NATO command on their territory as part of an international security presence," said a senior NATO diplomat. "Until we do, there will not be a deal."

Correspondents David Hoffman in Moscow and Daniel Williams in Belgrade contributed to this report.

CAPTION: Martti Ahtisaari will try to win a "firm commitment" from Milosevic.