SARAJEVO -- Bosnian Serb forces on Wednesday overran the last holdout of UN peacekeepers and thousands of refugees in the Srebrenica enclave, delivering a final mocking blow to UN authority.


Bosnian Serb military commander General Ratko Mladic arrived on the heels of his infantry at the UN base in Potocari, which had been the main headquarters of peacekeepers charged with protecting Bosnian civilians from Serb attack.


UN spokesman Alexander Ivanko said Bosnian Serbs were separating men from women and children among the approximately 40,000 terrified civilians who had gathered at the Potocari base after Srebrenica fell on Tuesday.


The Serbs said all males over 16 would be transported to nearby Serb-held Bratunac, where they would be "screened for war crimes," Ivanko added.


Up to 3,000 Bosnian civilians, mostly women, children and the elderly, were transported out of the enclave by Wednesday evening in two convoys of buses and trucks Mladic had brought with him, said another UN spokesman, Lieutenant Colonel Gary Coward.


They were bound for government-held Kladanj, about 45 kilometers west of the Srebrenica enclave, with some peacekeepers accompanying them "to ensure that the refugees don't suffer anymore," Ivanko said.


The convoys were to stop at the front line separating Serb- and government-held territory, from where the exhausted and terrified refugees would have to walk the final 16 kilometers, UN officials said.


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Angry demonstrators in Tuzla, another UN "safe area," meanwhile blocked a UN base in the northeastern city, preventing UN food and medical convoys from heading toward Kladanj, officials said.


Serbs asked the United Nations to supply fuel for the population transfer, though they refused all other UN assistance, including an offer of helicopters to evacuate about 70 sick and wounded, Coward said. UN officials then pressed the Serbs to allow them to carry out a medical evacuation by land.


The status of the remainder of the 400-strong peacekeeping contingency in Potocari was unclear. Coward said it appeared that some had been disarmed, but that quite a few peacekeepers inside the base still had their weapons.


"The Serbs are calling all the shots in Potocari," Ivanko said. "It definitely appears they wish to harass the refugees who are already terrified."


Bosnian Serbs marched into the village of Potocari, north of Srebrenica, earlier Wednesday, after surrounding it with tanks, multiple-rocket launchers and mortars aimed at the UN camp. A Bosnian Serb tank was parked outside the base's main gate, UN officials said.


Bosnian President Alija Izetbegovic demanded the resignation of Yasushi Akashi, the senior UN envoy in the former Yugoslavia, and the restoration of the "safe area" of Srebrenica. He also told reporters in Sarajevo it was not at all clear whether the Bosnian government would agree to an extension of the UN mandate in Bosnia past its November expiry date.


The number of Dutch peacekeepers held hostage by the Serbs rose to 42, after rebel forces overran two more UN observation posts Tuesday night and Wednesday, officials said. The Serb advances came after the Dutch~