PRISTINA, Yugoslavia (AP) _ Serb police let an ethnic Albanian food convoy pass through their Kosovo checkpoint and then shelled it, killing three people, a charity worker charged Tuesday.

The Mother Teresa humanitarian organization said the three were killed Monday near Malisevo, a former rebel stronghold in central Kosovo that Serb forces overran last month.

After the convoy of seven tractors passed, the Serbs opened fire, a spokesman for the U.N. High Commissioner for Refugees said, quoting survivor Mustafe Krasniqi.

A shell that landed on one of the tractors killed two aid workers instantly and mortally wounded a third, UNHCR spokesman Fernando del Mundo told The Associated Press.

The Serbs then raked Krasniqi's tractor with submachine gun fire, but he was unhurt except for minor burns, del Mundo said.

There was no immediate comment from Serbian authorities.

Del Mundo said both Krasniqi and local villagers claimed the convoy was deliberately targeted. Shells continued landing in the area for about 15 minutes after the one that killed the three aid workers, they said.

Tractors are commonly used to transport humanitarian aid and refugees in Kosovo, a southern province of Serbia. Though the aid vehicles were not clearly marked, they had registered with Serb police checkpoint as a humanitarian mission.

The charity, based in Kosovo, is responsible for the bulk of humanitarian assistance to the province. It is not affiliated with the Missionaries of Charity, the Roman Catholic order of nuns founded by Mother Teresa, who was an ethnic Albanian.

Serb police and the Yugoslav army continued shelling several villages southwest of Pristina in their drive to crush separatist resistance among Kosovo's majority ethnic Albanians.

The insurgent Kosovo Liberation Army is fighting to wrest Kosovo away from Serbia, the dominant republic in Yugoslavia. Ethnic Albanians outnumber Serbs 9-to-1 in the province.

The Albanians' Kosovo Information Center said three ethnic Albanians, including one woman, were killed Tuesday in Suva Reka. The center said Serb forces were burning houses in the town, following clashes Monday.

``We denounce the continuing violence in Kosovo which is forcing more people to flee,'' spokesman Kris Janowski of the UNHCR said Tuesday in Geneva. ``The situation is getting worse and worse.''

The fighting came as U.S. envoy Christopher Hill prepared to meet in Belgrade with Yugoslav President Slobodan Milosevic. After the meeting, Milosevic issued a statement demanding Washington lift its economic embargo against Yugoslavia.

Milosevic said the crisis in Kosovo can only be solved by peaceful means, but added that ``fighting terrorism presents a necessary condition for stabilization of the situation.''

UNHCR sent a 10-truck convoy Tuesday to refugees in the Baranski Do region, 35 miles west of Pristina. Humanitarian agencies estimate that as many as 50,000 refugees in Kosovo are living in the open.