Talks to implement the Kosovo peace plan collapsed early today after Yugoslav military leaders refused to endorse the terms of an agreement accepted last week by the Belgrade government. NATO officials responded by declaring they will launch an intensified round of airstrikes against Yugoslav targets.

The breakdown came after two days of talks in which NATO commanders had presented what they said were nonnegotiable instructions for the withdrawal of Yugoslav military and police units from Kosovo and the swift deployment of a peacekeeping force in the province. The Yugoslav delegation countered with its own proposal, at odds with the West both on the timetable of the withdrawal and the entry of the peacekeepers.

Standing outside a camouflaged hangar here where the two sides had haggled for 12 hours since early Sunday, British Lt. Gen. Mike Jackson, who led the NATO delegation at the talks, said the Yugoslav proposal "would not guarantee the safe return of all the refugees or the full withdrawal of Serb-led Yugoslav security forces."

"NATO therefore has no alternative but to continue and indeed intensify the air campaign until such time as the Yugoslav side is prepared to implement the agreement fully and without ambiguity," Jackson said. As the talks faltered Sunday night, air raid sirens sounded in Belgrade, but no strikes were reported. Within hours of Jackson's announcement -- and for the first time in several days -- antiaircraft fire was heard over the Yugoslav capital.

The two sides had no immediate plans to meet again. "The Serbs have our phone number," one NATO official said, explaining why Jackson did not want to continue the talks for a third day. "They can call us anytime they want to sign" the agreement as laid out by NATO. After the meeting, the Yugoslavs headed home by car and Jackson took a helicopter to another location in Macedonia.

Yugoslav Foreign Ministry spokesman Nebosja Vujovic said the Yugoslav delegation had pursued the talks "in good faith. . . . We are ready to talk further." He suggested that the Yugoslav resistance to the NATO plan was based on Belgrade's insistence that the U.N. Security Council approve a resolution authorizing a peacekeeping force before NATO troops are allowed to enter Kosovo, a province of Serbia, Yugoslavia's dominant republic.

Vujovic said the plan accepted last week in Belgrade by Yugoslav President Slobodan Milosevic called for "the deployment of an international security presence under the auspices of the United Nations or a presence established by a Security Council decision."

News of the breakdown in talks prompted President Clinton to cut short his weekend at Camp David and return to Washington late Sunday to monitor Kosovo developments.

A senior Pentagon official said that NATO is expected to "ramp up" the numbers of sorties over Yugoslavia and increase the number of targets to be struck today in response to the failure of the talks. NATO had continued bombing during the peace talks but had cut back on the number of bombing runs, particularly outside Kosovo. "We're going back to a full range of targets," the defense official said.

On Sunday, one NATO official had said that the Yugoslav military delegation -- led by two deputy chiefs of staff of the Yugoslav army, Gen. Svetozar Marjanovic and Gen. Blagoje Kovacevic -- had disagreed not just with the details of the Kosovo pullout plan but with the entire concept of the peace agreement. It spells out requirements that Yugoslav officials reveal the location of all mines, withdraw all air defense equipment and evacuate 40,000 troops from Kosovo within seven days.

Belgrade's motives remained uncertain, but Western officials said Yugoslav security forces appeared to be seeking more time to press an offensive against the separatist Kosovo Albanian guerrillas in the southern region of the province. The Yugoslav army has been waging fierce attacks near the Albanian border in the past two days, albeit at the same time attracting assaults by NATO warplanes.

On Sunday, for example, American B-52 bombers devastated Yugoslav artillery positions on a hillside near the Albanian border town of Morina, while other planes fired missiles at targets near the Kosovo towns of Suva Reka, Decani, Gnjilane, Prizren, Pec and Djakovica, according to Yugoslav officials and the state news agency Tanjug.

At the end of the first day of implementation talks on Saturday, officials on both sides were publicly upbeat. By late Sunday afternoon, however, one participant acknowledged that the mood had turned decidedly sour and said that Yugoslav delegates had developed what he called "a bad attitude."

According to Western sources, one dispute centered on NATO's determination to halt its airstrikes only after its reconnaissance aircraft have found evidence that a substantial withdrawal of Yugoslav forces from Kosovo was underway. As NATO spokesman Jamie Shea declared in Brussels Sunday, "the signature on the paper isn't enough to stop NATO's air operations; it is tanks going by border posts, or at least between Kosovo and Serbia, that . . . we are looking for."

But Yugoslav officials insisted on a different sequence of events that would begin with a bombing halt, the Western sources said. The officials also indicated that a complete troop withdrawal was possible only after the U.N. Security Council had approved a resolution setting out the terms of the peace accord -- a move that could occur sometime this week. But NATO has long rejected tying the Yugoslav withdrawal to a U.N. vote.

"It is standard Balkans and Milosevic blustering and huffing and puffing . . . an effort to wear us down," said one NATO official. He said that top alliance commanders were insisting on their own approach -- a withdrawal, then a pause in the bombing -- to ensure that Belgrade is not engaging in "lies and tricks" as its forces pull out.

The Yugoslav officers voiced many other complaints Sunday, according to NATO officials. They requested a total of 15 days to withdraw their forces instead of the seven offered by NATO, on grounds that bad roads and low fuel stocks made a greater speed impractical. They also complained about a NATO demand that all government forces withdraw into Serbia at least 15 miles from Kosovo's borders, and they argued that any troops that have lived in Kosovo should be able to remain there.

One sign of deepening Western anxiety about the potential breakdown of the accord was a decision by Finnish President Martti Ahtisaari, the European Union's envoy for Kosovo matters, to postpone a trip to Beijing for meetings with China's leaders about Kosovo.

The foreign ministers of the Group of Seven countries and Russia were scheduled to meet in Bonn later today to draw up a U.N. resolution that will, among other things, set up a command structure for a postwar security force in Kosovo.

Meanwhile, Defense Secretary William S. Cohen said that a number of non-NATO countries had agreed to dispatch troops for the force, including the United Arab Emirates, Jordan and Ukraine. But Russia might or might not be part of the operation, he said on the ABC News program "This Week." "It's up to the Russians at this point," he said.

Republicans in Congress continued to squabble about Kosovo. Sen. John McCain (R-Ariz.) said the GOP has hurt itself by adopting an isolationist view that is skeptical of overseas commitments. "I think we Republicans have to return to the traditions that have been the underpinnings of our party ever since the end of World War II," McCain said on the ABC show. "We have to be engaged in places where it is certainly an unpleasant experience."

NATO officials, meanwhile, said they could not readily draw conclusions about the Yugoslav military's intentions from monitoring recent activities in Kosovo. According to news service reports, intensified Yugoslav shelling along the Albanian border killed an 18-year-old woman and forced humanitarian workers to evacuate 1,000 people.

On the other hand, in possible signs of an impending pullout, NATO planes tracked the movement of a military train northward toward central Serbia from the southern Kosovo city of Urosevac and also photographed army and Interior Ministry police units using vans and trucks in the widespread looting of homes Friday and Saturday in the Kosovo city of Prizren. The latter evidence "gives us indications that troops may be trying to sort of help themselves before finally departing Kosovo," Shea said.

Even if a military agreement is signed, a host of other issues remain to be settled in discussions between Belgrade and U.N. humanitarian officials, including what kind of documentation will be required from returning Kosovo refugees and how to resolve cases that fall short of the requirement. Irene Kahn, who heads the office of the U.N. High Commissioner for Refugees in Skopje, the Macedonian capital, noted in an interview that the peace plan promised "the return of all refugees, and that has to be the bottom line, whatever mechanism is used."

Refugee officials said their offices in the Kosovo capital, Pristina, had been looted and damaged. Their challenge in reconstructing the rest of Kosovo can be gauged from a list of goods already stockpiled near the Kosovo border: 5,000 tents capable of sheltering 40,000 people, 75,000 blankets, 27,000 mattresses, 95,000 hygiene kits, 101 large potable-water containers and $10 million worth of door and window frames.

Kahn said the group's largest constraint at the outset will be the presence of large numbers of land mines planted along major roads. She said that NATO military officials are trying to get Yugoslav authorities to surrender their mine maps but that nothing had yet been produced. Moreover, NATO troops have promised to arrange for mine clearance only along main roads, not in rural areas. As a result, the refugee agency has printed 300,000 "mine awareness" leaflets in Albanian.

The U.N. refugee agency's first priority will be to provide basic aid to displaced people who never left Kosovo, a process that will likely take at least a month, Kahn said. Its second priority will be to arrange the return of refugees from camps and homes in neighboring nations. The return of those refugees already airlifted to Europe and the United States will have a lower priority, and they may spend the winter where they are.

In total, the agency is unwilling to promise that more than half of the nearly 1 million refugees who fled Kosovo will be returned this year. A firm timetable has not been drawn up, but no significant refugee returns to Kosovo are likely before mid- to late July, even if the military agreement is signed with no further delay, officials said.

Staff writers Charles Babington, John Mintz and Dana Priest in Washington contributed to this report.

CAPTION: British Lt. Gen. Mike Jackson, left, chief of the NATO rapid reaction force, walks with other soldiers to meet the Yugoslavs in Kumanovo, Macedonia. He called the their new peace proposal unacceptable.