In this story:

• Observers: Army fired over them
• Aid worker wounded in Albania
• NATO agrees to send protective force
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BELGRADE, Yugoslavia (CNN) -- A senior U.S. envoy is due to meet Yugoslav President Slobodan Milosevic on Monday to bring home the message that a cease-fire in Kosovo is threatened by clashes between Serbian forces and ethnic Albanian separatists.

The announcement of the meeting in Belgrade came as Western officials raised new concerns about the safety of peace monitors in Kosovo. On Sunday, the Yugoslav army reportedly fired a machine gun over a car containing U.S. diplomatic observers.

Yugoslav military authorities insisted, however, that the observers had mistaken backfiring vehicles for gunshots.

The alleged incident, which caused no injuries, came at a time of rising hopes for progress toward a settlement that would end months of violence in the restive Serbian province.

During talks with U.S. Balkans envoy Richard Holbrooke last month, Milosevic agreed to pull many of his police and troops from Kosovo. He also agreed to allow an international verification force to police the cease-fire after NATO threatened airstrikes against Serbian military targets.

But clashes between Serbian forces and Kosovo Liberation Army (KLA) guerrillas threaten a renewal of the fighting that killed more than 1,000 people and drove more than 250,000 of the province's residents from their homes.

U.S. envoy William Walker, who arrived in Kosovo last week to head a team of verifiers, will raise Sunday's alleged shooting incident during his talks with Milosevic, said Duncan Bullivant, spokesman for the verification mission.

"It's not why the meeting is taking place, but it will be raised at the meeting as an example of the sort of incident we would hope to see not happen in the future," Bullivant said.

"Speculative fire is hardly in the spirit of the cease-fire," he said.

Bullivant said it had been agreed that Walker would meet Milosevic in Belgrade at around 2 p.m. (1300 GMT) on Monday.

The talks were requested by Bronislaw Geremek, chairman of the Organization for Security and Cooperation in Europe (OSCE), which will oversee the future Kosovo Verification Mission.

"He (Walker) has been asked to meet Milosevic to raise concerns over the current level of violence in Kosovo and over the tit-for-tat nature of the violence," Bullivant said.

The clashes have raised concern for the safety of the unarmed observers, whose numbers are increasing daily in preparation for the launch of the 2,000-person verification mission next month.

Bullivant said Sunday's incident, involving a vehicle from the U.S. Kosovo Diplomatic Observer Mission (KDOM), was the first he was aware of and took place on the road between Pristina and the southwestern town of Prizren.

"The U.S. KDOM vehicle passed the VJ (Yugoslav army) column, the VJ opened up with a heavy machine gun, effectively firing over the top of the KDOM vehicle," he said.

Bullivant said the observers demanded an explanation from Serb authorities but received no response.

The state-run Yugoslav news agency Tanjug later quoted the Yugoslav army's Kosovo command as saying no shots had been fired but two army vehicles had backfired near the observers.

"It is obvious that the members of the U.S. mission in the armored car interpreted the exhaust pipe bangs as shooting," the agency quoted an army statement as saying.

In neighboring Albania, British aid worker Sally Becker -- well known for her work helping refugees during the war in Bosnia -- was shot and wounded on Sunday in the town of Bajram Curri, near the border with Kosovo, the OSCE said.

Becker was in Albania as head of the charity Operation Angel, a group of 26 women taking relief supplies to ethnic Albanian refugees fleeing the violence in Kosovo.

Bullivant said there were now 175 U.S. observers in Kosovo, 50 Britons, 15 French, 14 Russians and five Italians as well as 30 staff at the verification mission's Pristina headquarters.

NATO has agreed to deploy a military force in the former Yugoslav republic of Macedonia to rescue the verifiers if they come under attack.

Belgrade summoned the Macedonian charge d'affairs on Sunday to protest the deployment, state television said.

The verifiers are supposed to help implement a lasting peace settlement for Kosovo, but attempts to broker an autonomy deal for the province are hamstrung by ethnic Albanian demands for independence and Belgrade's insistence it remain part of Serbia.

Belgrade has responded coolly to draft peace proposals drawn up by U.S. mediator Christopher Hill, fearing they could let in independence for Kosovo by the back door.

Milosevic did not attend a planned meeting with Hill a week ago due to what officials said were other commitments.

Serbian President Milan Milutinovic offered on Sunday to host direct talks on a settlement in Pristina on Wednesday, but the initiative failed to win Western backing and looked set to be rejected by the ethnic Albanian side.

Reuters contributed to this report.