SARAJEVO, Bosnia-Herzegovina (AP) _ The U.N. commander for Bosnia failed Wednesday to solve a dispute with the Bosnian government that has left nearly 1,000 U.N. peacekeepers short of food and heat in bitter mid-winter.

Lt. Gen. Sir Michael Rose returned empty-handed from a trip to Tuzla in north-central Bosnia, where the United Nations has tried for a year to persuade the Serbs to allow aid flights to a nearby airport.

Government troops have been blockading 450 U.N. soldiers at the airfield and nearly 600 others in the region for eight days. U.N. officials angered the government by allowing a Serb liaison officer onto the airfield Jan. 8 as a guarantee against its military use.

The Serbs have continued to bar aid flights, prompting the government on Monday to give the United Nations until Feb. 1 to open Tuzla airport or withdraw.

The last U.N. helicopter flight to and from Tuzla airport was Jan. 11. The last fixed-wing plane landed in July 1994 when a Russian IL-76 was shelled on the ground while the crew tried to unload radar tracking equipment.

A U.N. spokesman in Tuzla, Capt. Philip Jerichow, said food was being rationed at the base and forward observation points were withdrawing from the nearly 240-mile front line because they were running out of provisions.

``There are cease-fire violations all along the confrontation line, but our soldiers cannot go out to the observation points,'' he said.

Another dispute lingered between Croats and Muslims, nominal allies against the Serbs in a U.S.-sponsored federation that has yet to take shape other than on paper.

A session of the federation assembly scheduled for Wednesday in Sarajevo was canceled after federation President Kresimir Zubak, a Croat, refused to step down and make way for Ejup Ganic, a Muslim who is also vice president of Bosnia's Muslim-dominated government.

Federation provisions call for rotating the post between Croats and Muslims every six months. The Croats argue that rotation can only begin half a year after election of the first president. Both sides agree elections cannot be held in wartime.

Dario Kordic, a Bosnian Croat leader, said Muslim political leaders lacked the goodwill necessary for the assembly to meet.

Kordic said help from Bosnian Croat allies abroad was needed to bridge the differences. The United States, Germany, Turkey and Austria are involved in behind-the-scenes talks to shore up the crumbling federation.

U.N. officials said Wednesday that the northwest Bihac region was generally calm, but noted troop movements that could signal more fighting.

A Dec. 24 cease-fire and subsequent four-month truce have failed to halt the fighting between Serbs and government troops around Bihac. The battles also involve Croatian Serbs and rebel Muslims not party to those agreements.

In Sarajevo, Serbs claimed the government had not fully honored a cease-fire provision for withdrawing its troops from a demilitarized zone on Mount Igman, a strategic height southwest of Sarajevo.

Joint inspection of the zone was canceled Wednesday because the Serbs wanted to land there and inspect outside the zone, and the government refused, U.N. spokesman Lt. Col. Patrick Declety said.

An estimated 200,000 people have died or disappeared since Bosnia's war began in April 1992, when Serbs rebelled against the republic's decision to secede from Serb-dominated Yugoslavia.