The U.S. head of peace verifiers in Kosovo, William Walker, on Saturday visited the site of a massacre where at least 35 ethnic Albanians were killed, describing what he saw as "horrendous".
Mr Walker said it appeared the victims had been shot execution-style and that whoever did it had no value for human life.
It's the worst killing spree since an October truce largely halted more than seven months of combat in the troubled Serb province.
The day after Serb forces launched a fierce assault on the area, the bodies of 35 people, many of them mutilated, have been discovered on a hillside in Kosovo.
The American chief of the monitoring mission run by the Organization for Security and Cooperation in Europe, was visibly shaken after touring the site just hours after the massacre.
Journalists and Kosovo Verification Mission representatives accompanied Mr Walker on his tour.
Villagers said Serb police had separated men from their families and led them toward the local police station.
Residents claim the Serb forces later turned and herded them up the hill, where they killed them.
Bodies apparently lay where they were slain, along cow paths and in deep, hilly ravines.
Serbian police claim dozens of mutilated bodies in civilian clothes found on the site were armed rebels killed in combat.
A police statement says what it calls "terrorists" were killed in an exchange of gunfire after they attacked police with automatic weapons and mortars.
It also says most of those killed wore uniforms with the insignia of the -- quote -- "terrorist K-L-A."
Yugoslav tanks and troops were part of the attack, pounding the area before fighting abated on Friday afternoon.
Verifiers and journalists heard villagers tell of a grisly massacre, but Serbian authorities didn't allow them to visit the site on Friday.
It was the worst killing spree since an October truce largely halted more than seven months of combat in the separatist province, and perhaps the most savage of the nearly yearlong conflict.
The informal ceasefire, which international officials have insisted is still largely intact, is now in serious danger of collapsing into a resumption of the province-wide fighting that devastated Kosovo in 1998.
More than a thousand people - mostly ethnic Albanians - have been killed since Yugoslav President Slobodan Milosevic launched an offensive last February to try to crush separatist militants and reinforce government control over the Albanian-majority province in Serbia.