In this report:

• NATO previously authorized air strikes
• Name perpetrators, Albright demands
• Warner wants U.S. forces in Kosovo

WASHINGTON (CNN) -- NATO will hold an emergency meeting Sunday in Brussels to discuss a response to the massacre of dozens of civilians in Serbia's rebel province of Kosovo, U.S. officials said.

International monitors have accused Yugoslav forces of murdering dozens of villagers in the southern part of Kosovo, where ethnic Albanians far outnumber Serbs.

The Organization for Security and Cooperation in Europe said U.S. diplomatic observers had counted 45 bodies. According to news reports, some had been mutilated. Eyes were gouged. Heads were smashed. One man lay decapitated in his courtyard. The victims included a young woman and a 12-year-old boy. Many were older men, including one 70-year-old.

All the victims were dressed in civilian clothing, despite Serb police insistence that most of them wore uniforms of the rebel Kosovo Liberation Army.

U.N., NATO and U.S. officials strongly condemned the killings, the worst killing spree in a nearly-year-long conflict between Serbians and Kosovo separatists.

United Nations Secretary General Kofi Annan said he was "shocked" to learn of the massacre.

"I am gravely concerned at this latest development and call for a full investigation by the competent authorities. I appeal once again to all sides in Kosovo to refrain from any action that would further escalate the tragic situation."

William Walker, the American head of the Kosovo Verification Mission, told a news conference the massacre violated pledges made by Yugoslav President Slobodan Milosevic in October to avert threatened NATO air strikes.

"There should be no doubt of NATO's resolve," a State Department spokesman said.

The alliance in October authorized its forces to hit Yugoslav military targets if Belgrade violated pledges to seek a peaceful solution to a conflict with ethnic Albanian separatists, but U.S. officials declined to discuss what options Washington would pursue at the NATO session.

United Nations chief war crimes prosecutor Louise Arbour will head a mission to Kosovo to investigate the massacre.

President Clinton strongly condemned the killings, which he blamed on Serbian security forces. "This was a deliberate and indiscriminate act of murder designed to sow fear among the people of Kosovo," Clinton said. "It is a clear violation of the commitments the Serbian authorities have made to NATO. There can be no justification for it."

U.S Secretary of State Madeleine Albright conferred with fellow NATO foreign ministers, including those from Britain, France, Germany and Norway. She demanded that Milosevic bring those responsible to justice, State Department spokesman James Rubin said.

Rubin said Albright was demanding that Milosevic "identify who gave the orders and who took this action, and ensure that those perpetrators are then brought to justice."

"We want the Serbs to cooperate in the investigation of this atrocity by the International War Crimes Tribunal," he added.

The head of an international observer mission visited the scene, near the village of Racak, and said it looked like an execution.

NATO Secretary-General Javier Solana warned that the killings would exacerbate tension in Kosovo, where the majority ethnic Albanians are fighting for independence.

Clinton said he was in contact with Solana and he urged both sides to work for a peaceful solution.

"It is urgent that these murders not trigger a spiral of reprisals," he said. "Both sides have a responsibility to work toward a peaceful resolution of this crisis and for a settlement that allows the people of Kosovo the self-government they so clearly deserve."

Sen. John Warner, the new chairman of the Armed Services Committee, said Saturday he favored putting a NATO military force in Kosovo as a deterrent.

"I think there should be some U.S. component to that NATO force," the Virginia Republican said. "If we leave that situation in Kosovo destabilized it will undo what we have achieved, although modest, in the last three to four years in Bosnia," Warner added.

Independence-minded ethnic Albanians outnumber Serbs by nine to one in Kosovo, a Serbian province where some 2,000 people were killed and 250,000 driven from their homes in fighting last year.

A statement issued by Serbian police said they had killed several dozen guerrillas after they came under attack while trying to find the murderers of a policeman. It said an investigative team had been blocked on Saturday by the monitors who told them that they should do their job without a police escort, explaining they feared a new clash.

Last autumn, villagers accused Serb police of massacring two groups of ethnic Albanians. Belgrade said the killings had been staged by the ethnic Albanians to goad NATO into threatening air strikes against it.

The United States brokered a cease-fire in October, but violence has escalated sharply in recent weeks.

At the NATO meeting, U.S. officials said the United States will propose the following agenda:

- Full cooperation from Yugolav authorities, including Slobodan Milosevic, in identifying the perpetrators of the massacre and in working with the international tribunal in The Hague.

- Strong language stating the Serbs are not complying with the cease-fire agreement fashioned in October.

- Language stressing the Serbs must comply with the Kosovo verification mission.

The Associated Press and Reuters contributed to this report.