Serbs Block Evacuation of Wounded Civilians : Bosnia: Gunmen defy U.N. and their own commanders in stopping convoy bound for besieged enclave of Srebrenica.

Serbian gunmen defied U.N. authorities and their own commanders Tuesday by blocking a medical mercy mission trying to evacuate wounded civilians from Bosnian territory under rebel siege for nearly a year.

A convoy of eight buses converted to makeshift ambulances was stopped by armed Serbs as it tried to cross into Bosnia-Herzegovina at the town of Zvornik, on the border with Serbia, a spokeswoman for the U.N. high commissioner for refugees reported in Belgrade.

The ambulances were trying to reach the embattled Bosnian enclave of Srebrenica, where 60,000 people are encircled and holding out against Serbian forces. Ham radio operators in the enclave said Serbian forces were shelling the town, and Serbian military communiques reported advances by their troops.

Echoing an excuse repeatedly offered to thwart aid missions, local Serbs in Zvornik said they know nothing of their leadership’s promises to allow the evacuation teams unhindered passage.

Only a day earlier, the Bosnian Serb military strongman, Gen. Ratko Mladic, had assured U.N. authorities that his forces would not interfere with efforts to rescue the most severe casualties from last week’s battles, in which Serbs overran the Bosnian town of Cerska.

Conditions in Srebrenica and nearby Konjevic Polje are so desperate that 20 to 30 wounded people are dying each day from poor sanitation and lack of medical supplies, the World Health Organization in Geneva has reported.

A doctor for the U.N. agency who managed to gain access to the eastern Bosnian war zone over the weekend described the state of hospitals in the government-held pockets as appalling and more primitive than any he had encountered in Third World countries racked by conflict, including Afghanistan and Liberia.

The doctor, Briton Simon Mardel, reported that medical workers had only plastic bags with which to dress wounds and that anesthetics and disinfectants had not been available for months.

Prompted by the shocking reports, the commander of U.N. forces in Bosnia, French Gen. Philippe Morillon, visited Srebrenica on Monday and obtained promises from Mladic that his forces would allow relief workers to evacuate the worst cases.

“There’s some confusion about the agreement,” U.N. refugee agency spokeswoman Lyndall Sachs said of the blockade imposed against the convoy at Zvornik.

Swedish first-aid workers driving the ambulances planned to spend the night on the roadside and resume negotiating with the local warlords today. But Sachs indicated she doubted that there would be any change in the Serbs’ position.

The medical evacuation team had planned to take about 75 wounded Bosnians, mostly Muslim Slavs, from the wretched clinics in Srebrenica and Konjevic Polje to the city of Tuzla, a well-defended government stronghold better equipped to treat the casualties.

Upon hearing of the blocked evacuation effort, a World Health Organization official in Geneva announced that emergency medical supplies will be prepared and delivered to U.S. forces for the next airdrop of relief goods over the area.

American military cargo planes have been parachuting parcels of food and medicine to eastern Bosnia in nightly sorties since March 1 to get around the Serbian rebel roadblocks aimed at starving non-Serbs into surrender.

Serbs have conquered 70% of Bosnia-Herzegovina since taking up arms nearly a year ago in anger at the republic’s secession from Serbian-dominated Yugoslavia. The rebels’ siege, bankrolled by Belgrade, is aimed at wresting as much Bosnian territory as possible for annexation to a Greater Serbia.

Srebrenica is one of the few eastern Bosnian cities still under the Sarajevo government’s control, which has made it a magnet for those displaced by the Serbian practice of “ethnic cleansing” by which non-Serbs are forced out of their homes at gunpoint.

While foreign relief workers and Bosnian Serbs engaged in the standoff at Zvornik, new cracks emerged in the facade of Serbian commitment to the nationalist cause in this increasingly violent and volatile Serbian capital.

More than 1,000 people massed at the site of a deadly clash with Serbian police two years ago to protest the dictatorial leadership of Serbian President Slobodan Milosevic.

Witnesses said protesters riled by the arrest of a popular singer hurled rocks at police and plainclothes security agents who had infiltrated the crowd.

One woman suffered a broken leg when she and other demonstrators were run down by a police van.

The demonstration in the heart of Belgrade marked the second anniversary of a massive student protest March 9, 1991, against Milosevic’s control of the media and the nationalist hysteria that triggered Yugoslavia’s violent breakup.

More than 150,000 people are dead or missing as a result of the Serbs’ revolts against Muslim and Croatian plans to secede from Yugoslavia, and more than 2 million have been made homeless by the fighting and by “ethnic cleansing.”