The rebel Serbs, who have been advancing steadily on the U.N.-protected Muslim enclave for four days, then issued an ultimatum, warning that Srebrenica must be cleared of all people within 48 hours.

The ultimatum applies to more than 40,000 mostly Muslim refugees, government army troops and all U.N. peacekeepers, said U.N. spokesman Lt. Col. Gary Coward, who termed the nationalist

Serb challenge completely unacceptable.

Advertisement

If Srebrenica falls to the Bosnian Serbs, it will mark the first loss of one of six government-held pockets created to give refuge to tens of thousands of Muslims driven from their homes in the brutal ethnic warfare that has ravaged Bosnia for more than three years. It will represent another blow to U.N. credibility and promises to unleash a humanitarian disaster.

U.N. officials said they considered calling in warplanes from the North Atlantic Treaty Organization to back up the Dutch peacekeepers in their clash with Bosnian Serb troops, but declined to do so.

On Sunday, the top U.N. officials in the Balkans, in a letter to Bosnian Serb army commander Gen. Ratko Mladic, warned air strikes would be called if the Serb rebels attacked the Dutch force, an elite team positioned along the southern perimeter of Srebrenica to block the nationalist Serb offensive.

Whether the U.N. can stop the Bosnian Serbs from overrunning the U.N.-designated safe areas has become the latest test of the beleaguered, crippled peacekeeping mission, whose future is under debate now in the world’s capitals.

The U.N. is determined to protect the enclave, Coward said Monday night.

But the mission has seen its ability to fulfill any of its mandate, from guarding safe havens to delivering humanitarian aid, eroded steadily by Bosnian Serb defiance and instructions from the U.N. political leadership to calm and not inflame the tense Bosnian situation.

Through much of Monday, Bosnian Serbs fought with government troops off the southeast corner of the Srebrenica enclave and pounded the town with artillery and tank fire, U.N. officials said.

Advertisement*

They also battered Zepa, a second U.N.-protected enclave, with heavy machine gun and mortar fire. A U.N. observation post manned by Ukrainians was attacked.