BOSANSKI BROD, Yugoslavia (AP) _ Hundreds of frightened people with no faith in official word of a cease- fire tried to flee this town in northern Bosnia-Hercegovina Sunday.

They were trying to make their way to the comparative safety of neighboring war-torn Croatia. But in a sign of the rising tensions that threaten to embroil Bosnia's Muslims, Serbs and Croats in civil war, militiamen turned back males of fighting age who lacked permission to leave from the local Croat-Muslim militia, known as the Patriotic League.

''We are mobilizing everyone we can to defend our city,'' said a militiaman, who would only identify himself as Bahrudin.

Bahrudin sported a green patch bearing the Muslim crescent, and his beret was adorned with the ''U'' symbol of Ustashas, the fascist regime that ruled Croatia during World War II and massacred Serbs in Croatia and Bosnia- Hercegovina.

War in Bosnia would be far bloodier than the fighting that has killed up to 10,000 people in Croatia.

At least 40 people have died in the past week around Bosanski Brod, an industrial town of 33,000 that is 42 percent Croat, 35 percent Serb and 12 percent Slavic Muslim.

Some of the victims were Serbs villagers the Serbs say were massacred by Croats and Muslims on Thursday night.

On Friday, Serb militiamen and Serb-dominated federal army troops shelled Bosanski Brod, Croats and Muslims said.

The oil refinery and the main department store were hit. Serbian stores have been smashed and plundered.

Serbs sealed off their part of town by puting metal spikes across the road.

Bosnian leaders negotiated a cease-fire late Saturday, but an unidentified man cycling near the buffer zone was killed on Sunday by a sniper's bullet.

Luka Vukoje, a 27-year-old Croat militiaman, died at the hands of another sniper in the nearby village of Kolibe.

''What kind of people are they?'' wailed Nikola Curic, Vukoje's uncle, as he waited in the line of hundreds of cars to cross into Croatia.

''We are afraid of the Chetniks, we now know they are capable of committing any crime,'' said Ana Maric, a 60-year-old Croat, referring to extreme Serb nationalists.

''I'm not fleeing. But I want to save these things,'' said Nikola Kljajic, motioning to a tractor-trailer loaded with a freezer, refrigerator, stove, pots, pans, bedding and a bicycle.