SARAJEVO, Bosnia-Herzegovina (AP) _ U.N. officials today reported increased military activity in northern Bosnia, including a mortar attack that knocked out electricity in Serb-held Brcko.

The shelling intensified as Bosnian Serbs and the Muslim-led government hardened their positions before a crucial meeting Friday of Russian, American and West European foreign ministers hoping to push the warring sides back to the negotiating table.

Meanwhile, five Croat civilians were killed in a U.N. protected area near Daruvar, 48 miles north of Zagreb, Croatian police said. The incident late Wednesday occurred in an area jointly controlled by U.N. forces and Croatian police about 12 miles from a Serb-held part of the sector.

A group of Serbs crossed into the Croat area and accosted eight Croatian men who were walking near the village of Batinska Rijeka, the police said. They said five Croats were immediately shot and killed. Three fled, two of whom were wounded on the run.

U.N. officials also reported that a U.N. military observer died today and another observer and a government soldier were injured by an explosion while on patrol north of Sarajevo. It was not clear whether their vehicle struck a land mine or was hit by an artillery shell.

U.N. military observers reported five mortar shell explosions Wednesday evening around Brcko, 70 miles north of Sarajevo in the narrow part of an east-west corridor vital to Serb supply lines.

Two shells hit a transformer station, cutting electricity to the town, said U.N. spokesman Cmdr. Eric Chaperon. It was not clear how long the outage lasted or how wide an area was affected.

Chaperon said U.N. observers could not tell who fired the shells. They have confirmed earlier shelling of the town by Muslim-led government troops, including a mortar attack Tuesday that killed at least three civilians and wounded more than a dozen.

Reported military buildups by both sides around Brcko have fed speculation that a major battle could be looming. Loss of the corridor would be a blow to Serbs because it is their only land link to their holdings in western Bosnia and parts of Croatia.

Chaperon also reported shelling in other chronic toublespots in the north, including the Olovo-Kladnje area about 25 miles northeast of Sarajevo, where another troop buildup has been reported.

Friday's meeting of foreign ministers in Geneva could mark a turning point in efforts to settle the 2-year-old Bosnian conflict.

Western and Russian frustrations are peaking, with France threatening to withdraw thousands of its peacekeepers from Bosnia and President Clinton under pressure from Congress to lift an arms embargo on the Bosnian government.

The foreign ministers' quest is complicated by hardened demands from the warring parties.

Bosnian Serb leaders Tuesday reiterated their reluctance to part with conquered territory and said they would not resume talks unless there is an overall cease-fire.

But the Muslim-led government and Bosnian Croats, who recently ceased hostilities, staked claims to 58 percent of Bosnia for a federation they are forming in an accord brokered by Washingon.

Bosnia's Muslim president, Alija Izetbegovic, said Serb withdawal from ''occupied territories'' was a condition for resuming negotiations.

The Serbs, who control 70 percent of Bosnia, are certain to reject that - especially after the Muslims brashly shelled Brcko on Tuesday while Bosnian Serb political leaders were meeting there.

The shelling killed three people, including a pregnant woman.

Bosnian Serbs, armed by the Yugoslav federal army, launched the war in April 1992 when they rebelled against secession from Serb-dominated Yugoslavia. More than 200,000 people have been killed or reported missing.

Full negotiations to end the bloodshed have been suspended since February.

The Muslim-led government, meanwhile, appears increasingly to adopt tactics long used by the Serbs in the conflict.

It had long been the government, not the Serbs, who demanded a total and permanent cease-fire before talking peace. Now the government opposes such a deal, fearing Serbs would use it to freeze their holdings in place.

By shelling Brcko when top Bosnian Serbs were meeting there - then accusing Serbs of shelling themselves to win world sympathy - the government also appeared to be mimicking a common Serb tactic.