SARAJEVO -- Surging government forces pushed back Serb troops in northwest Bosnia and threatened a strategic Serb supply road south of Sarajevo, U.N. peacekeepers reported Monday.


In apparent retaliation, Serb unleashed the deadliest shelling attack on Sarajevo-area civilians since February, killing two people and wounding at least seven in the suburb of Hrasnica late Saturday, the U.N. said.


The barrage of mortar rounds violates a ban on heavy weapons around Sarajevo, and could prompt NATO air strikes. But Lieutenant Colonel Tim Spicer said the UN, because of the difficulty of pinpointing mortars from the air, might use other weapons to enforce the ban if the shelling continued.


The fiercest combat raged in northwest Bosnia as the government army's Fifth Corps continued a two-pronged offensive out of the Bihac region that has gained some 250 square kilometers, the biggest government victory of the war.


One prong of the attack had reached Kulen Vakuf, 35 kilometers south of Bihac. Soldiers in the other prong renewed an assault on the Serb-held town of Bosanska Krupa, which they surrounded Saturday.


The Bosnian Serb leader, Radovan Karadzic, tried to boost Serb morale Sunday night at a rally in Bosanski Petrovac, near the new front line south of Bihac. He said a counterattack would be launched within days to recapture lost territory.