SARAJEVO, Bosnia-Herzegovina (AP) _ Thousands of civilians and hundreds of Dutch peacekeepers huddled in shelters Thursday when Bosnian Serbs unleashed a barrage of shells on the eastern U.N. ``safe area'' of Srebrenica.

In Sarajevo, another U.N.-designated area supposedly off-limits to combat, at least six civilians were killed and 11 were wounded by mortar fire.

The peacekeepers' main base at Potocari, in the northern part of the Srebrenica enclave, was hit by at least 10 shells, and a Dutch observation post on Srebrenica's southeastern front was damaged by Bosnian Serb tank and mortar fire, said U.N. spokesman Alexander Ivanko.

None of the 450 Dutch peacekeepers was hurt, but two civilians were killed and an unknown number were wounded, said Lt. Col. Gary Coward, another U.N. spokesman. Bosnian radio reported four dead and 17 wounded and predicted the casualty count would rise once the shelling subsided long enough to allow the victims to be collected.

At least one shell reportedly hit a refugee center in Srebrenica, but there were no immediate details. The enclave is home to an estimated 42,000 Bosnian Muslims, most of them refugees.

Coward said the Serb rebels also attacked the enclave's northern and southern perimeters. The heaviest fighting occurred near a U.N. observation post that overlooks the main north-south road through Srebrenica, which is about 50 miles east of Sarajevo.

``The Serbs appear to be intent on seizing high ground around where the road enters the enclave,'' Coward said.

Occasional mortar fire from both sides pounded Sarajevo's suburbs Thursday. At least six civilians were killed and 11 were wounded in government-held parts of Sarajevo, Bosnian health officials said.

The Serbs stepped up shelling of U.N. enclaves after the government launched an offensive June 15 to break the Serb siege of Sarajevo.

U.N. officials were considering whether the Dutch peacekeepers at the Srebrenica post should withdraw, as they were forced to do last month when a nearby post came under Serb attack.

Srebrenica should have a contingent of 730 peacekeepers, but since April the Serbs have refused to allow the United Nations to bring in 300 replacements for a unit that was permitted to leave.

Serbs have also denied passage to U.N. supply convoys to the enclave, forcing peacekeepers to survive on basic rations.

There have been unconfirmed reports that the Dutch are planning to pull out of Srebrenica in September. But sources say on condition of anonymity that the United Nations is having trouble finding replacement troops acceptable to the Serbs, who oppose peacekeepers from Islamic countries.

Meanwhile, a U.N. relief convoy carrying flour into Sarajevo via the only available land route arrived Thursday morning under cover of fog, which apparently prevented nearby Serb gunners from targeting it.

Once the fog cleared, however, Serbs fired anti-aircraft shells and machine-gun rounds at two armored vehicles using the perilous route over Mount Igman, southwest of the city, Landale said. There were no casualties or damage.

Carl Bildt, the European Union's mediator to the conflict, met with U.N. commanders and Bosnian government leaders in Sarajevo on Thursday. Bildt is trying to convince Bosnia's Muslim and Croat allies to allow a 12,500-member European rapid reaction force into Bosnia to help protect peacekeepers.

The government apparently fears the European force might interfere with troop movements or its clandestine efforts to equip its army in defiance of a U.N. arms embargo. The Serbs also oppose the force, which they perceive to be anti-Serb.