BELGRADE, Jan. 16 -- At least 45 ethnic Albanians were found slaughtered today in the village of Racak in the breakaway Serbian province of Kosovo in what appeared to be the bloodiest spree in the year-long conflict between the warring sides. International observers -- including U.S. Ambassador William Walker, the head of the multinational force attempting to monitor the tenuous cease-fire -- immediately accused Serbian security forces of mass murder. President Clinton issued an angry condemnation of the killings. NATO announced it would hold an emergency meeting on Sunday in Brussels to discuss what action to take, and there were demands for access for investigators of the U.N. war crimes tribunal. The bodies of the ethnic Albanians, which included a young woman and a 12-year-old boy as well as a number of older men, were discovered scattered on a hillside and heaped together in a gully near Racak, 15 miles south of the provincial capital, Pristina. Villagers said Serbian forces had rounded up the victims, taken them up a hill and killed them. Some had their eyes gouged out or their heads smashed; many were old men shot in the head at point-blank range. U.S. diplomatic observers counted 45 bodies, but local villagers and the Kosovo Liberation Army said they had counted 51, including a 3-month-old baby. That report, however, could not be confirmed. It was the highest single death toll in the 11 months since the Serbian crackdown on ethnic Albanian separatists began. Kosovo is a province of Serbia, the dominant Yugoslav republic. A cease-fire, brokered by U.S. envoy Richard C. Holbrooke with Yugoslav President Slobodan Milosevic, has been in effect since Oct. 12. The truce halted more than seven months of fighting between the majority ethnic Albanian population and Serbian authorities. After touring the grisly scene and seeing the bodies of some 20 ethnic Albanians scattered across a hillside, a visibly shaken Walker called the killings "an unspeakable atrocity," and "a crime very much against humanity." Walker said he did not "hesitate to accuse the government security forces of responsibility." He urged prosecutors from the International War Crimes Tribunal for the Former Yugoslavia to investigate. Clinton responded to the news with a strong condemnation that suggested the Serbs had violated commitments made to NATO to forestall Western military action. "This was a deliberate and indiscriminate act of murder designed to sow fear among the people of Kosovo," Clinton said in statement. "It is a clear violation of the commitments the Serbian authorities have made to NATO. There can be no justification for it." The State Department announced the emergency NATO meeting, and spokesman James P. Rubin said: "There should be no doubt of NATO's resolve." Rubin said Secretary of State Madeleine K. Albright had been in contact with other NATO foreign ministers and demanded that Milosevic bring those responsible to justice. Louise Arbour, the chief prosecutor at the U.N. war crimes tribunal in The Hague, said she would lead an investigation into the massacre in the next few days. Her spokesman, Christian Chartier, said Arbour was demanding "immediate and unimpeded access." The prosecutor's efforts to lead a team to investigate earlier charges of atrocities in Kosovo have been rebuffed by Serbian authorities, who do not recognize the jurisdiction of the tribunal and refused to issue visas. "For this, we don't care," said Chartier in a telephone interview. "We are no longer prepared to discuss jurisdiction." He said Arbour and her team planned to leave for Kosovo within 48 hours. Diplomats in Belgrade said the massacre had dashed any remaining hopes of preserving the already shaky cease-fire. But they dismissed the likelihood of NATO intervention in the short-term, saying the alliance was divided and unwilling to commit the ground troops needed to enforce a peace agreement. NATO Secretary General Javier Solana said in Brussels that NATO "will not tolerate a return to all-out fighting and a policy of regression in Kosovo." U.N. Secretary General Kofi Annan, who said he was shocked and "gravely concerned," called for a full investigation. "There have been problems and violations of the agreement for many weeks now, and clearly it is shaky," Rubin said. "It is not as bad as it was last fall, but it's heading rapidly in that direction," he said. Ethnic Albanians outnumber Serbs by nine to one in Kosovo, where more than 1,000 people were killed and tens of thousands driven from their homes in fighting last year. The massacre occurred two days after the Kosovo Liberation Army (KLA) freed eight Yugoslav soldiers, ending a five-day standoff. Serbian police, who were backed by tanks, said in a statement that they had sealed off Racak on Friday while searching for the killers of a policeman, but had been shot at by KLA rebels. "In the clashes, several dozen terrorists, most of whom were in uniform with KLA insignia, were killed," the statement said. But residents said the police officers had separated men from their families and told them they would be taken to the nearby town of Urosevac. Instead, a group of more than 20 men were led up a hill and executed, villagers said. Walker said, "We were hoping for a reciprocal confidence-building measure. Instead we have this. . . . the killing of innocent civilians." CAPTION: Relatives mourn over the bodies of ethnic Albanians discovered scattered on a hillside and heaped in a gully near the Kosovo village of Racak. Victims reportedly included a 12-year-old boy. ec CAPTION: Ethnic Albanian boy weeps upon recognizing relative slain in Kosovo. ec