SARAJEVO, Bosnia, July 18 -- Bosnian Muslims resisted tank and artillery attacks by Serb forces on the eastern Zepa enclave today and threatened to use Ukrainian U.N. peacekeepers as human shields if NATO fails to take action.

The hard-pressed Ukrainians -- 79 troops entrusted with the defense of the U.N. "safe area" and its 17,000 Muslim inhabitants -- were under threat from both sides as attacking Serbs threatened to kill a group of eight at an observation post they were surrounding and to target the peacekeepers' base if NATO jets are called in.

With Zepa resisting capture exactly one week after separatist Serbs captured the nearby U.N.-designated "safe area" of Srebrenica, Western governments were still at an impasse over how to tackle the crisis.

Bosnian government troops have surrounded the Ukrainian U.N. compound north of Zepa, U.N. spokesman Alexander Ivanko said in Sarajevo. "They said they would use Ukrainian peacekeepers as human shields unless NATO airstrikes are called in against advancing Bosnian Serbs," he said.

NATO jets flew over Zepa on Sunday but did not strike.

Serb tanks and artillery resumed attacks up the ravine leading to Zepa this morning but were apparently still stuck one mile away. "The attacks are going on. But Bosnian soldiers are successfully resisting them, and there is no change in the lines," Fadil Heljic, a Zepa amateur radio operator, said. In a radio message late Monday, he said: "We have casualties, a lot of casualties. . . . The humanitarian situation here is horrifying. Almost no medicine is available."

The United Nations estimates the government can muster 600 to 1,000 fighters in Zepa. Some are using armored personnel carriers seized from the Ukrainian peacekeepers on Monday.

Volodymyr Yelchenko, a Ukrainian government official, urged the United Nations to withdraw his country's peacekeeping troops from Zepa and Gorazde. "The Ukrainian troops are unarmed and completely defenseless. If they are not immediately withdrawn, a huge tragedy will ensue," he said. Bosnian government troops in Gorazde stripped weapons from the Ukrainian troops Sunday, assaulting their commander and leaving the peacekeepers defenseless, the United Nations said. CAPTION: A refugee from Srebrenica holds her son while an aid worker prepares a vaccination at a camp in Tuzla. CAPTION: German soldiers arrive at San Damiano air base in Italy to participate in NATO operations in Bosnia.