LJUBIZDA, Yugoslavia (AP) _ In a field in southwest Kosovo, several mounds of raw earth are surrounded by police tape. A human thigh bone and a child's shoe lie nearby, and blue plastic tarp covers what seems to be rotting clothing.

Though no peacekeeping troops or international forensic experts were available at Ljubizda on Thursday, the site appeared to be the mass grave announced hours before in London by British Foreign Secretary Robin Cook.

Cook offered no details, beyond saying 50 bodies were found buried in a garbage dump at Ljubizda, a village southwest of Pristina.

Abedin Ademi, 67, a farmer whose field borders the area, said the grave contained bodies of ethnic Albanians killed in April by Serb forces.

``The Serbs dug a deep hole and put Albanians in it _ men, women and children _ and dropped in a grenade. They buried those who were killed and those who were injured, even if they were still alive,'' said Ademi, acknowledging that he was relaying hearsay information.

Such finds feed anti-Serb sentiment among Kosovo Albanians, and have led to a rash of violence against the dwindling Serb minority in Kosovo.

The head of Kosovo's former ethnic Albanian guerrillas on Thursday urged an end to revenge-taking against Serbs and called on them to return to the province.

Emerging from a meeting in London with Cook, Kosovo Liberation Army leader Hashim Thaci appealed to Kosovo Serbs to return ``because we are interested in establishing in Kosovo a multiethic society.''

His comments were sure to be ignored by Serbs in Kosovo and elsewhere. They blame the KLA for the wave of anti-Serb violence since Yugoslav President Slobodan Milosevic's troops left the province in June as part of a peace agreement, in return for an end to NATO bombing.

Most of Kosovo's prewar population of 200,000 Serbs have fled since then, amid dozens of killings and abductions and other violence directed against them to avenge the deaths of more than 10,000 ethnic Albanians during the 18-month crackdown by Milosevic's forces.

In Belgrade, deputy Serbian Premier Vojislav Seselj accused the United States of being allied with Kosovo Albanian ``murderers, drug traffickers and terrorists'' wreaking violence on Kosovo Serbs.

Yugoslav army and Serb police forces should return peacefully to Kosovo ``as soon as possible,'' he said, adding: ``If there are no peaceful means available, we shall have to find other ways.''

In the Kosovo capital of Pristina, KLA commander Agim Ceku said the demilitarization of his forces is almost complete, more than two weeks before the Sept. 19 deadline agreed to by the KLA as part of the Kosovo peace deal. KLA fighters have been turning in heavy weapons, but Ceku said his forces would not completely disarm.

``Disarmament and demilitarization are two completely different words,'' he told reporters after meeting with German Defense Minister Rudolf Scharping.

The KLA is permitted to keep rifles and other small arms under the peace agreement, but Ceku also has gone on record saying that the KLA will form Kosovo's new army _ something not foreseen in peace terms, as the province legally remains part of Serbia.