The political leader of the Kosovo Liberation Army said today that the ethnic Albanian rebel group is committed to building "a modern civil society" in the Serbian province, and he appealed to fleeing Serbs to return to live in "a democratic Kosovo" as long as they have not committed any crimes.

Hashim Thaqi, a onetime rebel field commander who was named head of a provisional ethnic Albanian government in Kosovo this spring, made his first official appearance in this provincial capital since the end of the war, symbolizing the shift in the balance of power just one day after the last Serb-led Yugoslav forces pulled out.

"Today, a new era will start in Kosovo," Thaqi said at a news conference after signing a KLA-NATO agreement that requires the guerrillas to store most of their weapons and reduce their military profile. "We are not interested in building a criminal society, but a modern civil society," he said.

KLA members have routinely intimidated Serbs in Kosovo and in recent days, while the demilitarization plan was being negotiated, rebels detained and beat a number of Serbs they accused of taking part in atrocities against ethnic Albanians. The rebels' actions have contributed to an exodus of tens of thousands of Serbs from Kosovo to Belgrade and other cities in central Serbia. Today, however, Thaqi implored the frightened Serbs to come back.

"The phenomenon of Serbs leaving is [a] phenomenon that is upsetting to us," Thaqi said. "We ask all the Serbs who left and haven't done any crimes to come here and live in a democratic Kosovo." Belgrade officials, NATO commanders, the head of the Serbian Orthodox Church, and now the KLA leadership have all urged Serbs who left Kosovo to return.

Meanwhile, as NATO troops continued to fan out across the Connecticut-size territory, the alliance suffered its first fatalities of the 10-day-old peacekeeping mission. Two British soldiers serving with the 69th Gurkha Field Squadron were killed this afternoon when an explosive device went off as they were searching a schoolyard in the village of Negrovce, 20 miles west of Pristina. Two civilians also died in the blast, and a third was injured, NATO officials said.

NATO spokesmen have said that Kosovo was heavily mined and booby-trapped by both Yugoslav and KLA forces during the 2 1/2-month-long campaign by the Belgrade government to purge the province of its ethnic Albanian majority.

KLA officials today began providing NATO commanders with maps of their mine fields as part of the demilitarization plan. Yugoslav army commanders turned over details of their mine emplacements in the first days of their withdrawal, which concluded Sunday when the last of 40,000 army troops and Serbian special police deployed in Kosovo pulled out.

The conclusion of the Yugoslav military withdrawal prompted NATO on Sunday to announce an official end to its air campaign against Yugoslavia -- an offensive that lasted 78 days -- and today the Serb-controlled Belgrade government announced it would ask the federal parliament to lift the state of war it imposed as the first NATO bombs fell on March 24. In Washington, Defense Secretary William S. Cohen ordered the return to the United States and European bases of more than 300 of the 730 U.S. aircraft that participated in the NATO air campaign.

Under the KLA demilitarization plan, the rebel leaders agreed to turn in all of their heavy weaponry to NATO authorities for storage within the next 30 days; to give up all small arms, except pistols and hunting rifles, within 90 days; and to "refrain from all hostile or provocative acts."

Even as NATO and the KLA leadership announced details of the plan, however, it was clear that disarming the rebels will be difficult to monitor and verify. In addition to the problems of determining whether all weapons are turned in, the plan is certain to meet opposition among lower-ranking rebels, who suffered heavy casualties in their 18-month secessionist war against government forces.

Many rebels may be unwilling to abandon the group's military role -- let alone their arms -- without being assured they will ultimately achieve their goal of an independent Kosovo. Kosovo is a province of Serbia, the larger of Yugoslavia's two republics. NATO favors autonomy for Kosovo within Serbia and Yugoslavia.

The demilitarization guidelines include provisions -- such as the possible creation of a territorial military force that rebel officials liken to the U.S. National Guard -- that make NATO and U.N. officials here uneasy. And the rules for disarming KLA members conflict with the practice already established by some NATO peacekeepers here, particularly U.S. forces, of confiscating all rebel weapons.

Demilitarization of the KLA is a central objective of NATO as it seeks to end the violence in the province and was a provision of the U.N.-backed peace plan that led to withdrawal of Yugoslav and Serbian forces from Kosovo and the introduction of the NATO-led peacekeeping force.

Thaqi's decision to back the plan prompted a congratulatory telephone call this morning from President Clinton. According to a senior U.S. official, Thaqi impressed the president by speaking about the need for ethnic harmony in Kosovo and his opposition to reprisal attacks by ethnic Albanians against Serbs.

On Sunday, a patrol of French NATO forces stood by while ethnic Albanian villagers looted and burned the Serbian village of Grace in northern Kosovo the night after its Serbian inhabitants fled. The NATO troops took no action to stop the pillaging -- the ethnic Albanians said they were reclaiming possessions the Serbs had stolen -- until reinforcements arrived.

"It was an incident I regret," Lt. Gen. Mike Jackson, the British commander of the NATO peacekeeping force, said today. "We fell short of what I wanted." Today, NATO forces escorted a caravan of about 2,000 Serbs who had fled Kosovo back into the province.

The six-page KLA demilitarization plan is to be phased in over the next three months. Effective immediately, KLA members are not to carry weapons "of any type" within a mile of Yugoslav force assembly areas, main roads and towns or external Kosovo borders.

The KLA has agreed, within the next four days, to "close all fighting positions, entrenchment and checkpoints on roads, and mark their mine fields and booby traps."

Within seven days, KLA forces are required to establish "secure weapons storage sites," which will be verified by NATO officials. Over the next three months, KLA members must turn in all weapons except pistols and hunting rifles at those sites.

At the end of the accord, the KLA proposes the "formation of an army in Kosovo along the lines of the U.S. National Guard." When asked to address the proposal, State Department spokesman James P. Rubin called it "an expression of the aspirations of the KLA" and added: "This is a decision for the future."

Correspondent R. Jeffrey Smith in Skopje, Macedonia, contributed to this report.