CELIC, Bosnia-Herzegovina (AP) _ A bomb destroyed a bridge that Muslims use to cross into Serb territory on Friday, a day after gunmen killed a Muslim farmer cutting hay in a field in Bosnian Serb territory.

The attack on farmer Alija Agic had Muslims in the countryside north of Tuzla vowing revenge. Russians in the NATO-led peace force in the region stepped up patrols to prevent more violence.

Agic, 67, was working Thursday with a friend in his field near Celic, in Bosnian government territory about 15 miles north of Tuzla.

Two cars pulled up, and several men got out and hid in brush, said the friend, Malil Salihovic. The gunmen fired repeatedly, striking Agic in the chest. He died at his home a few hours later.

``He was shot on his own land,'' said his 59-year-old widow, Zifa. ``I wish there were no more world at all.''

Agic's field lies just over the boundary separating the Muslim-Croat Federation and Bosnian Serb territory under the Dayton peace accord. The peace agreement bans all weapons from the border.

It was believed to be the first killing of Muslims on Bosnian Serb territory since late April, when two Muslim refugees trying to reach their homes in Bosnian Serb territory crossed a mine field near Doboj.

On the same day the farmer was killed, two men with automatic weapons confronted another Muslim from Celic working on a field on the Serb side of the line, said Andrea Angeli, a spokesman for U.N. police in Tuzla.

He was taken to a Serb police station and beaten, Angeli said. Russian troops in the area secured his release, and took him back to Celic, Angeli said.

No one was arrested in the killing or in Friday's bombing of the bridge near Serb-held Doboj.

No one was injured in the explosion.