Serbian forces backed by artillery reportedly closed in Wednesday on two more towns in eastern Bosnia, and a U.N. aid convoy was blocked by Serb officials for a second day.

Also Wednesday, six U.S. Air Force cargo planes dropped 42 tons of food and medical supplies over Srebrenica, the 10th mission of the American relief effort.Serbs were taking aim at two towns, meanwhile, Srebrenica and Konjevic Polje.

The last Muslim defense position outside Srebrenica fell to advancing Serbs who were within two miles of the town, said local Bosnian army spokesman Ibrahim Becirevic, reached by ham radio from Zagreb, Croatia.

A Bosnian Serb military statement issued Tuesday said Serb forces seized two strategic hills about six miles north of Srebrenica.

A U.N. convoy headed to the Konjevic Polje area near Srebrenica to evacuate up to 75 sick and wounded Muslims was blocked for a second day by local Serb commanders at Zvornik on the Serbian border, said Tony Land, the representative in Sarajevo of the U.N. High Commissioner for Refugees.

He said the main dispute with the Serbs focused on who could be evacuated. The Serb commander insisted soldiers could not be included, but Land said that would exclude males age 18 to 60, a "large proportion" of the seriously wounded.

U.N. land convoys to eastern Bosnia have been repeatedly turned back by Serbs, prompting the United States to start dropping aid from the air last week.

U.N. refugeee officials were negotiating with Serbs to secure passage for the trucks and an aid convoy on Thursday to Srebrenica, where a World Health Organization doctor reported 97 people were close to death.

Muslims were trapped in Konjevic Polje after Serbs routed them last week from nearby Cerska.