Serbs Block Food Delivery to U.N. Force : Bosnia: Rebels demand withdrawal of government troops near Muslim enclave. Humanitarian aid crisis deepens.

Stepping up pressure on desperately needed aid convoys, the Bosnian Serb army Monday blocked food delivery to U.N. troops in a starving Muslim enclave and conditioned its resumption on the withdrawal of advancing government forces.

U.N. spokesman Alexander Ivanko said that ultimatum, made by Bosnian Serb army commander Gen. Ratko Mladic, “is extremely outrageous.”

It also has become typical here. Aid convoys, on which tens of thousands of Bosnians rely for their sustenance, are routinely blocked or turned back by Bosnian Serbs. Food warehouses in this besieged capital are empty now for the first time in the 38-month war.

Delivery of humanitarian aid is the crux of the United Nations’ original mission in Bosnia-Herzegovina; that even this task is no longer being accomplished points to the increasing impotence of the U.N. presence.

A food convoy arrived in Sarajevo on Monday for the first time in three weeks, but the United Nations had to rely on Bosnian government pickup trucks to transport the supplies over Mt. Igman, a narrow road within easy range of rebel Serb artillery gunners. The food was symbolic--enough flour to bake a day’s ration of bread.

The humanitarian aid crisis is deepening as U.N. officials announce they are reverting to basic “peacekeeping principles” and returning to the “status quo” that existed before NATO air strikes on Bosnian Serb positions led to the capture of nearly 400 U.N. peacekeepers 2 1/2 weeks ago.

The new mandate, issued over the weekend, was effectively the first formal admission that all gains made by peacekeepers in the last six months have been wiped out by Bosnian Serb defiance and U.N. capitulation.

“Basically, a lot of shots are being called by the Bosnian Serbs,” Ivanko said Monday, conceding that the 4,000 U.N. troops in Sarajevo have nothing to do now but to run anti-sniper patrols, “monitor the situation” and wait for instructions from the U.N. Security Council.

Among the concessions that Bosnian Muslims consider the most alarming is an agreement to allow Bosnian Serbs, instead of U.N. peacekeepers, to escort future convoys trucking in supplies for Sarajevo as they pass through rebel Serb-held territory.

An aid airlift, which provided Sarajevo with more than half its food, was shut down two months ago when Bosnian Serbs said they would shoot at the planes. Then they halted almost all convoys after the May 26 bombings by the North Atlantic Treaty Organization.

An important test of a Serbian promise to permit convoys comes today when the United Nations will try to move one caravan into government-controlled Sarajevo and a second one into a Bosnian Serb-controlled part of the capital. The Muslims will receive 200 tons of food; the Bosnian Serbs, whose population is a third of the Muslims’, will receive 100 tons.

But given Bosnian Serb treatment of convoys thus far, U.N. officials were skeptical Monday. Today’s convoys will have Bosnian Serb escorts, and there was concern over whether the trucks will even reach their destinations.

Over the weekend, two convoys with oral permission from the Bosnian Serbs left Belgrade for two Muslim enclaves in eastern Bosnia, but they were turned back, said Mark Cutts, head of the Office of the U.N. High Commissioner for Refugees, which sponsors most of the convoys.

Another convoy arrived at the Muslim enclave of Zepa but was held up for three days while rebel Serbs alleged it was smuggling weapons. On the third day, the Bosnian Serbs claimed they found ammunition hidden under some flour sacks. They commandeered the food and delivered it to a rebel Serb-held town.

On Monday, Mladic blocked a convoy that was to deliver rations to British and Ukrainian peacekeepers in the enclave of Gorazde and demanded that Bosnian government troops withdraw. Late Monday, the convoy was still stranded outside Gorazde, U.N. officials said.

No relief convoy has made it into the enclaves in nearly a month, Cutts said.

Besides food, the Bosnian Serbs have cut gas, electricity and water to Sarajevo, and fuel in the eastern enclaves is dangerously low, U.N. military sources say.

People in Sarajevo are still eating. They harvest vegetables from gardens, and plenty of food is smuggled through a tunnel under the airport. But it is expensive.

The U.N. refugee agency estimates that it needs 60 10-truck convoys a month to feed Sarajevo’s 280,000 residents when the airport is closed, as it is now.

The Bosnian Serbs also suddenly halted visits of the International Committee of the Red Cross to the 144 hostages who remain in Serbian custody, Red Cross spokeswoman Nina Winquist said Monday.

Meanwhile, senior U.N. officials in Zagreb, Croatia--headquarters for the U.N. Protection Force--confirmed that Gen. Bernard Janvier, commander of all U.N. forces in the former Yugoslav federation, has been in contact with Mladic.

“Mladic has asked Gen. Janvier some specific questions [about] when certain foodstuffs are going to be exhausted,” one official said.

That such a senior official has had any contact with Mladic, critics said, seemed to contradict the U.N. pledge not to negotiate with those whom it was labeling terrorists shortly after the peacekeeper-hostages were first captured.

The senior U.N. officials also said the new European rapid-reaction force will be available to help move relief convoys into Sarajevo and other needy areas of Bosnia.

But they placed conditions on when it will be called on and sought to dispel expectations that the force--expected to be approved this week by the Security Council--will somehow alter the balance of power between the warring sides.

Times staff writer Dean E. Murphy in Zagreb contributed to this report.