The 10-truck convoy carrying 90 tons of food and medicines set off at dawn for Cerska, where the more than 100 people are reported to have died recently from starvation. In 10 months of fighting, Serbian attackers have not let any aid get to the village, where a peace- time population of only 5,000 is reported to have quadrupled with the arrival of refugees.
But after an eight-hour wait for Serbian permission on the Bosnian Serb border, UN convoy chiefs were forced to give up, and turned back. Local Serbian commanders claimed fighting was too intense to let the food trucks pass. Frustrated UN aid chiefs vowed they would try again today. The Serbs denied passage to the UN convoy despite a face-to-face confrontation between the UN refugee chief, Jose Maria Mendiluce, and the Bosnian Serb leader, Radovan Karadzic, at his Pale headquarters, near Sarajevo, over the Serbian tactics of trying to starve out their Muslim enemies.
Mr Mendiluce, earlier in Belgrade, declared he would present Mr Karadzic with a timetable of aid convoys that the UN intended to use to help up to 100,000 trapped Muslims in eastern Bosnia this week. 'We will not negotiate,' he vowed.
But Bosnian Serb leaders continued to stall, adding fresh conditions before the UN convoys could cross Serb-held territory. Ljubisa Vladusic, a Bosnian Serb minister, insisted on a truce in eastern Bosnia coming into force first. 'We have nothing against aid supplies for the Muslims in the Drina valley, but an end to the fighting is a precondition,' he told Mr Mendiluce.
The crisis over the worsening plight of half a dozen Muslim enclaves in eastern Bosnia has been brewing for weeks, after Serbs began systematically turning back UN food convoys to Muslim strongholds in the region.
The latest Serbian refusal to allow food to Cerska places Bosnia's Serbian leaders on a collision course with UN aid chiefs. The insistence on a truce being agreed first is a Serbian delaying tactic - throughout the war Bosnian Serb commanders have used skirmishes to stop food and medicine reaching their Muslim enemies.
Cerska lies 40km (25 miles) east of Zvornik, a town on the Bosnian Serb frontier. Under the international peace plan brokered by Lord Owen and Cyrus Vance, the whole region is supposed to go back to Muslim control. But the Serbs have no intention of handing back territory they seized, which is why local sieges round the remaining Muslim enclaves in the area have tightened.
Six thousand Muslims fleeing Cerska reached Muslim-held Tuzla recently, some in a state of acute malnutrition, and suffering such severe frostbite that their fingers and toes had to be amputated. Cerska has become a symbol to Bosnia's embattled Muslims. The failure of this convoy could affect the future of the entire UN aid mission to Bosnia.
The Cerska convoy is seen as a test of the UN's ability to stand up to the Serbs. Bosnian Muslim suspicions that the UN is not up to the task have already reduced relations to an all-time low.
In Sarajevo, the city council is refusing to distribute UN aid supplies until the organisation gets food through to the enclaves in the east. Mr Mendiluce added Sarajevo to his itinerary yesterday, urging local leaders to end a boycott that is primarily hitting the city's already under-nourished 380,000 civilians.
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