SARAJEVO, Bosnia-Herzegovina (AP) _ With a new escalation of fighting in northwest Bosnia, a U.N.-brokered truce seemed dangerously close to collapse Sunday without clearing the first of many hurdles.

Bosnian Serbs, clearly defying the terms of the accord, halted the movement of U.N. military convoys through much of their territory, while refusing to open a road out of Sarajevo.

A shell that may have been fired from Serbs in neighboring Croatia killed a 19-year-old girl Sunday morning at a school in Bihac, in the northwest. Another shell killed a 15-year-old girl and wounded her mother. Government radio said a total of 11 people were wounded when dozens of shells hit.

The shelling, plus a mortar attack on a bridge in downtown Bihac that killed five people Saturday, may have been a response to an attempt by government troops to push out of the encircled town, said Col. Gary Coward, a U.N. military spokesman in Sarajevo.

Bosnian government troops appear to have seized Klokot, a village two miles northwest of Bihac and about a mile east of the Croatian border, Coward said.

``To fire on civilians in the very crowded town of Bihac, where 60,000 people live, is nothing less than murder,'' said Paul Risley, another U.N. spokesman.

The bloodshed in Bihac put new strains on a cease-fire that started on Dec. 24 and on the overall four-month cessation of hostilities agreement signed Dec. 31 by Bosnian Serbs, the Muslim-led government and Bosnian Croats. Serbs in neighboring Croatia and rebel Muslims aligned with the Serbs did not sign.

The agreement was aimed at giving negotiators time to bring the warring parties back to the bargaining table. More than 200,000 people have died or disappeared since the war began in April 1992, when Serb nationalists rebelled against a decision by the government to secede from Yugoslavia.

The capital and most other fronts have been quiet for three weeks. But many U.N. officials believe this is only a pause in a war that neither side is ready to end.

The four-month truce agreement called for the Serb side to permit civilians and U.N.-authorized aid convoys to cross the airport from Sarajevo to government-held areas to the west _ a small but vital step toward a peace settlement.

The Serbs reneged on a promise to open the route Saturday. Lt. Gen. Michael Rose, the U.N. commander, met with Serb leaders at their headquarters east of Sarajevo on Sunday in an attempt to change their minds.

But Rose and U.N. civil affairs chief Viktor Andreev made no headway.

Andreev said that while the government insists on the road opened to all traffic including commercial shipments, the Serbs consider it open only to the United Nations and other international humanitarian organizations.

One U.N. official predicted the truce would collapse unless the route were opened. Opening the road would give the Bosnian government an incentive for peace, by giving it the ability to better supply its capital of 280,000 people.

Further raising tensions, the Bosnian Serb military commander, Gen. Ratko Mladic, sent a letter to Rose on Saturday saying U.N. military convoys could no longer pass through Serb territory to the eastern government enclaves of Srebrenica, Gorazde and Zepa.

The letter, a copy of which was provided to The Associated Press, said heavy snowfall had caused road conditions to deteriorate. It told Rose to cease troop movements in and out of the enclaves.

``We find the rationale behind (the letter) rather dubious,'' said Risley, the U.N. spokesman. The United Nations provides fuel to Serb snowplows to clear the roads, ``so obviously this is not a problem,'' he said.

U.N. supply and redeployment convoys are scheduled to move into each of the enclaves once or twice a week.

Convoys bringing aid to the enclaves weren't affected by Mladic's letter.

Government troops, meanwhile, were harassing the U.N. operation in Tuzla, in eastern Bosnia. They have blockaded the Tuzla air base to demand the withdrawal of a Serb liaison officer stationed there since Jan. 8.

In what appeared to be the first war-related casualty in Sarajevo since the cease-fire started, a 13-year-old boy, Igor Krstic, was shot in the abdomen while riding his sled on a snow-packed hillside Saturday.

Authorities weren't sure whether it was a stray bullet or a sniper attack.