Bosnia's latest cease-fire, barely a day old, was in danger of unraveling Sunday when the Muslim-led government claimed Bosnian Serbs had joined Croatian Serbs and rebel Muslims in attacking its troops in the northwestern enclave of Bihac.

The Muslim-led government threatened to resume fighting and asked for an urgent session of the U.N. Security Council to consider the situation.Bosnian Serb Gen. Ratko Mladic filed his own complaint with the United Nations, saying Croat troops from Croatia and Bosnia were attacking his forces north of Livno, the Bosnian Serb news agency SRNA reported.

In Sarajevo, the cease-fire appeared to be holding. Children played in the snow, pulling sleds up a hill past bullet-riddled cars and apartment blocks blasted by tank, artillery and small arms fire during Bosnia's 33-month war.

The government ultimatum said: "Attacks in the north of the Bihac pocket must cease by Dec. 26 or the Bosnian government will be forced to launch attacks elsewhere to reduce the pressure."

U.N. spokesman Gary Coward said: "The Bosnian government filed a protest with us this morning alleging that Bosnian Serb forces were redeploying from positions around Bihac town to join in attacks on Velika Kladusa."

Aid worker Jerrie Hulme reported from the pocket that shooting around Kladusa was sporadic and less than Saturday but said there were "lots of maneuvers, soldiers moving into position . . . There was a lot of troop movement."

Croatian Serbs and rebel Muslim forces armed by them are not parties to the cease-fire which took effect Saturday.

U.N. commander Gen. Sir Michael Rose described the situation around Bihac as "extremely confused" and said his peacekeeping troops were trying to verify whether Bosnian Serbs were involved in the fighting.

Bosnian Vice-President Ejup Ganic told Reuters: "The only place in the country where we are under real pressure is Bihac. If we can't get a real cease-fire there we're not going to take one at all. We will wait until tomorrow for the U.N. to act."

Ganic made plain that talks on a more comprehensive cessation of hostilities envisioned under the cease-fire would take place until the problem in Bihac was settled to government satisfaction.

Bosnian government troops have been fighting to defend the besieged Bihac pocket from a combined assault of Bosnian Serb, Croatian Serb and rebel Muslim troops.