SARAJEVO, Bosnia-Herzegovina (AP) _ The United Nations shut down the Sarajevo airlift today because Serb rebels, angered by the government's arrest of a Serb on spy charges, have closed aid routes.

The Muslim-led government said Friday it was holding an ethnic Serb who works for the U.N. High Commissioner for Refugees on suspicion of spying.

A Bosnian government statement accused Svetlana Boskovic, 29, of direct involvement in the ``kidnapping'' of Sarajevo residents trying to pass Bosnian Serb checkpoints in U.N. vehicles.

U.N. relief officials demanded Boskovic's unconditional release. They have been in contact with her, but have not been allowed to see her since she was arrested Thursday, said UNHCR spokeswoman Kris Janowski. Boskovic's detention appeared to be in retaliation for the arrest two weeks ago of a Muslim journalist by Bosnian Serbs.

Serbs reacted to Boskovic's arrest by announcing they were closing aid routes into Sarajevo. That, in turn, led UNHCR to suspend aid flights today.

The agency said there wasn't enough storage capacity at the airport, and that with the route closed into the city, it couldn't transport humanitarian aid to needy Sarajevans.

The dispute over detention of Boskovic highlighted growing tension between the Bosnian government and Serbs six weeks into what is supposed to be a four-month cease-fire. International mediators have been unable to use the truce, as hoped, to get the two sides back to the negotiating table.

Meanwhile, fighting in the northwestern Bihac pocket Friday was among the most serious violations of the truce that took effect Jan. 1. Government troops and Bosnian Serbs were fighting over a plateau overlooking Bihac town, a U.N.-declared ``safe area'' where civilians are supposed to be under U.N. protection.

U.N. officials said the fighting was close to but did not directly endanger civilian areas in the mostly Muslim enclave. They said the Serbs might be trying to take control of the town's water supply system.

U.N. spokesman Maj. Herve Gourmelon said government forces appeared to have retaken on Friday territory lost the previous day.

Fighting near Bihac has stymied U.N. efforts to start talks to end nearly three years of war, in which more than 200,000 people are dead or missing. Most of the Bihac fighting has pitted Muslim-led government forces against rebel Serbs from nearby Croatia and rebel Muslims, neither of which signed the truce.

Bosnia's war started when minority Serbs revolted against a vote by Muslims and Bosnian Croats to secede from Serb-dominated Yugoslavia in 1992.