Fourteen Serbian farmers harvesting hay were killed in bursts of automatic gunfire Friday night within earshot of British peacekeeping troops in the worst single attack against Kosovo's beleaguered Serbian civilians since NATO forces entered the province six weeks ago.

NATO officials said they had no suspects in the killings, which they described as an organized assault, and acknowledged that they are all but powerless to stop a continuing plague of ethnic violence. "It's a hard truth," said British Lt. Gen. Mike Jackson, the NATO commander in Kosovo. "I regret these deaths bitterly, but . . . we can't be everywhere all the time."

The killings provoked outrage among the few hundred Serbian villagers in Gracko, and defiance from Yugoslav President Slobodan Milosevic, who called for the return of the Yugoslav Army and Serbian police to Kosovo, a province of Serbia, the dominant republic of Yugoslavia. In condemning the killings, NATO and United Nations officials urged Serbs in Kosovo not to join a exodus that has already slashed by three-quarters the province's prewar population of 200,000 Serbs, about a tenth of the total population.

"I see your tears. Your pain. And I beg your pardon," said Bernard Kouchner, the U.N. administrator for Kosovo, who visited Gracko this afternoon with Bishop Artemije of the Serbian Orthodox Church in an attempt to persuade the villagers to remain. "We failed to protect you."

Villagers blamed the rebel Kosovo Liberation Army for the killings and said they believed the gunmen came from a neighboring ethnic Albanian village, one of several nearby damaged by Serbian forces during NATO's bombing campaign against Yugoslavia. The KLA's political leader, Hashim Thaqi, denied rebel involvement in the shootings and said the KLA would assist international investigators.

"This act of violence, this madness, has been condemned by all of us," said Thaqi. "It is an act that has damaged the already difficult situation in Kosovo. . . . I use this opportunity again to call on Serbs not to leave Kosovo and not to be afraid of Albanians."

However, a senior U.N. official said today that some evidence pointed to the involvement of a local KLA faction. He said the attack could preface an internal KLA struggle between hard-liners who oppose surrendering weapons and those who are willing to work closely with the West. The official said that Thaqi visited U.N. officials today and seemed genuinely stricken by the incident.

A spokesman for Louise Arbour, the chief prosecutor of the international war crimes tribunal, said she would assert her authority over the investigation into the killings and that its perpetrators could be brought before the tribunal at The Hague. Two investigators from Arbour's office went to the morgue in Pristina, Kosovo's capital, to examine the bodies today. They plan to visit Gracko tomorrow.

In Gracko, home to about 80 Serbian families, the sound of keening women carried from houses today, and men shedding angry tears paced through the village in groups, glaring at reporters and international peacekeepers.

The shooting occurred around 9:30 Friday night in fields on the outskirts of the town, 10 miles south of Pristina. The farmers were gathering hay and corn when 13 of them were herded onto a dirt track between two fields and shot at close range. A 14th man was shot as he sat on his tractor. According to villagers, the ages of the dead ranged from 15 to 60.

A British peacekeeping unit on patrol nearby heard the gunfire and discovered the bodies.

"They were just bringing in our grain, said Zivane Nderkovic, whose two sons were killed in the ambush. "We are murdered and the world only helps Albanians."

"We will retaliate," shouted one man before being led away by other villagers. "It is only a question of when."

International and local leaders called for calm and pleaded with the dwindling number of Serbs here not to flee the province in fear.

"Brothers, don't go," said the Bishop Artemije. "We were born here. We live here. And here is where we should stay."

According to villagers, only 200 of the 450 people who lived in Gracko remained after NATO entered the province early in June, and across Kosovo the Serbian exodus is even more pronounced. The departures to other parts of Serbia continue daily as the desperation of the Serbian population increases.

Since the arrival of NATO peacekeepers, at least 28 Serbs have been killed, a dozen Serbian or Gypsy homes are burned daily, and at least 20 Serbian Orthodox churches have been damaged or destroyed, according to Serbian and NATO reports.

Tensions have also been high for weeks in the Lipljan area where one ethnic Albanian was arrested for firing a shotgun into a Serbian house, and Serbs responded by leaving hand grenades in a marketplace. They failed to detonate.

Today, some ethnic Albanians in the vicinity said the killings in Gracko were understandable.

"These killings are very bad," said Sadri Luma, 37, who lives across the fields from Gracko, which is just south of the town of Lipljan. "But people from that village looted and burned during the war. All the killing should be stopped, but it's hard to convince the desperate people who lost families and homes."

In a statement issued from Belgrade, the capital of Serbia and Yugoslavia, Milosevic accused NATO of tolerating "bands of murderers" in Kosovo and called for the return of Yugoslav Army and Serbian police to the province. The United Nations "must enable the urgent return of the police and Yugoslav army contingent as stipulated under the military-technical agreement," Milosevic said.

Under the military agreement between NATO and Yugoslavia that ended 11 weeks of NATO bombing, a limited number of Yugoslav soldiers and police were to be allowed back to Kosovo. NATO has shown no inclination to implement that part of the accord.

Villagers, for their part, said they wanted NATO to send Russian peacekeeping troops to protect them. Gracko residents said they asked NATO troops for protection in the fields when the harvest began seven day ago, but their pleas were ignored.

"How can we be protected by the armies that bombed us," said Lukio Backo, 58, pointing at British and Canadian forces, which patrol this area. "It's impossible."

Correspondent R. Jeffrey Smith contributed to this report.