SARAJEVO, Bosnia-Herzegovina (AP) _ The government demanded the withdrawal of 450 peacekeepers from a base in northeast Bosnia, a top official said today, signaling a sharp deterioration in ties with the United Nations.

The government's minister for relations with the United Nations, Hasan Muratovic, told The Associated Press he sent a letter Monday to U.N. officials demanding the pullout from the airport at Tuzla.

The Muslim-led government is angered by the failure of the United Nations to open the airport and the U.N. decision to allow a Serb liaison officer there.

Bosnian army troops have blockaded 450 U.N. troops at the airport for the last week, along with some 600 other peacekeepers in the Tuzla area. Most of the airport troops are Norwegian.

``This letter is an ultimatum. We will not change our position,'' Muratovic said. He said airport contingent should begin pulling out by Feb. 1 and complete the pullout by March 1.

Tuzla is the largest city outside Sarajevo in Bosnian government hands. The U.N. commander there, Gen. Gunnar Ridderstad, said he regarded the letter as pressure to open the Tuzla airport.

The Bosnian government has long sought to have the airport open to U.N. and aid flights, and turned it over to the United Nations last year for that purpose. Serbs demanded a liasion officer to ensure that the government did not use the airport for military purposes.

``Despite even the presence of the Bosnian Serb army liasion officer at the base now, Bosnian Serb authorities continue to refuse our request to have this airport open,'' said U.N. spokesman Paul Risley in Sarajevo. Serb artillery remains within striking range of the air field.

The order to withdraw from Tuzla airport was unlikely to cause more fighting directly, but it was a sign of ill will that will make implementation of a four-month cease-fire more difficult.

``The blockade isn't aimed at us,'' Ridderstad said, ``but we are suffering from it.''

He said land telephone lines were cut off and soldiers were conserving food and doing with less heat.

Another 600 Scandinavian soldiers are blockaded in a base north of Tuzla, but neither they nor Pakistani peacekeepers farther south are apparently affected by the government's order.

More than 200,000 people have died or disappeared since Bosnia's war began in April 1992, when Serb nationalists rebelled against a decision by Muslims and Croats to secede from Yugoslavia.

Bosnia's combatants made new promises earlier today to cooperate with the truce, even as they appeared to be sliding toward resuming their war.

U.N. officials said the Muslim-led government promised to eliminate one sticking point _ presence of its troops in a demilitarized zone southwest of Sarajevo.

Bosnian Serbs allowed several U.N. convoys to reach besieged Muslim enclaves in eastern Bosnia. They had revoked permission during the weekend.

The U.N. response was cautious. ``Both parties have not been delivering on their promises,'' said U.N. spokesman Lt. Col. Gary Coward.

Meanwhile, more fighting was reported in northwest Bosnia, the main trouble spot since the planned four-month cease-fire went into effect Jan. 1.

Coward reported heavy small-arms and mortar fire around the town of Velika Kladusa along the Croatian border. About 25 miles to the south, there was fighting near the town of Bihac.

Government troops in the Bihac enclave been resisting attacks by Serbs from Croatia and Bosnia, as well as rebel Muslims.

The Bosnian truce was aimed at giving negotiators time to reassemble peace talks.