U.S. and Russian negotiators struggled late tonight to resolve the last key differences over the size and shape of Moscow's zone of responsibility within the NATO-led peacekeeping force in Kosovo.

After a long day of intense discussions at the Finnish presidential palace, U.S. Defense Secretary William S. Cohen and Russian Defense Minister Igor Sergeyev announced that they were prepared to work through the night to try to seal an agreement outlining the nature of Russia's participation in the Kosovo force by early Friday.

Secretary of State Madeleine K. Albright and Russian Foreign Minister Igor Ivanov also met here today but have left the main negotiating on military matters to the defense ministers. The four officials were expected to gather in a final session Friday morning if the all-night session bears fruit.

Cohen said that while they fell short of reaching agreement today, the United States and Russia have managed to resolve "a lot of outstanding issues." Sources close to the talks said the sticky question of command and control has been all but settled in a way that will preserve NATO's insistence on a unified command structure while allowing the Russians to say their soldiers will not have to take their orders from NATO officers.

What remained to be settled, they said, were the exact contours of the geographic area that Russian soldiers would patrol. The emerging formula would allow the Russians to serve in sectors controlled by the United States, France and Germany; in addition, Russia would be allowed to maintain a presence at Pristina airport, where 200 Russian troops have blocked access by NATO peacekeepers who want to establish their command headquarters there.

Speaking in Paris, President Clinton told reporters that the Helsinki talks had "made pretty good progress," adding, "The next day or so I think we'll get it worked out."

"We had a very, very good partnership with the Russians [concerning peacekeeping forces] in Bosnia, and I think once these agreements are made, people on all sides will keep all the commitments, and we'll have a good partnership here," the president added.

The talks have dragged on in part because Albright and Cohen are striving to keep leading NATO allies informed of the negotiations. The alliance has divided Kosovo into five sectors controlled by the United States, France, Britain, Italy and Germany, and U.S. officials said that any deal to implant a Russian presence on the Kosovo military map will require careful scrutiny by participating nations.

Albright and Cohen have scheduled a meeting in Brussels Friday afternoon with the North Atlantic Council, the alliance's top decision-making body, so that all of NATO's 19 governments can be fully briefed.

The slow pace of the NATO consultations was said to have frustrated Ivanov, who at one point wanted to break off the talks and fly back to Moscow. But after further consultations with Moscow, Ivanov and Sergeyev agreed to stay on. "We are in the final dash, and we are concentrating our efforts in order to achieve an agreement as we were instructed by our two presidents," Sergeyev said.

Cohen and Sergeyev insisted they were determined to reach a deal so their respective presidents, Clinton and Boris Yeltsin, could formalize the agreement when they meet Sunday in Cologne, Germany, at a summit conference of the Group of Seven major industrialized democracies plus Russia.