SARAJEVO -- Croatian army and Bosnian government troops struck back at Serbs in northwest Bosnia on Friday, trying to regain ground four days before a U.S.-brokered truce is to take effect.


As fighting jolted the peace process, the U.S. State Department said that high-level talks -- announced by U.S. President Bill Clinton on Thursday along with the truce agreement -- will now take place on Oct. 30 or 31, not Oct. 25 as originally announced.


Spokesman Nicholas Burns said the date was pushed back at the request of Croatian President Franjo Tudjman, whose country is holding elections on Oct. 25.


The Croatian-Moslem counteroffensive against the Serbs was not unexpected -- fighting often has surged before previous truce deadlines in last-minute skirmishing for position.


UN officials said 3,500 Croatian soldiers were in Bosnia or on its border with Croatia, providing artillery support and reinforcing positions for a counterattack that Bosnian government troops had launched south of Bosanska Krupa, near the Croatian border.


In nearby Bihac, UN regional commander Colonel Erik Dam said by telephone that the joint counteroffensive had halted a Bosnian Serb advance about 40 kilometers west of Bosanska Krupa.


"The front line is stabilized," Dam said. "With reinforcements from Croatia, the government forces are able to halt the Bosnian Serb attacks."


UN officials had initially reported 100 Croatian soldiers in the region Thursday.


Croatian troops were crucial to a joint government-Croat offensive that won back wide swaths of territory from the Serbs during recent months. Croatia is not a party to the cease-fire, but Richard Holbrooke, the U.S. envoy who brokered the truce, said Thursday that Croatian participation has been "indispensable" in reaching it.


UN spokesman Jim Landale reported other clashes overnight around Serb-held Doboj, a key rail junction 100 kilometers north of Sarajevo, where hundreds of explosions were registered.


Even if fighting ebbs however, other developments could delay implementation of the cease-fire, which is due to start after midnight Tuesday.


Electricity and gas utilities must be restored in Serb-besieged Sarajevo by the time the truce goes into effect. U.S. officials have acknowledged the Tuesday deadline might have to be stretched if utilities are not restored by then.


William Eagleton, the UN special coordinator for Sarajevo, said mines infesting the area around a main power pylon near Sarajevo needing repair would have to be removed before utility crews could start working on it.


"We hope they have a map and can remove the mines." said Eagleton. "If they don't know where they are it could take some time."


Another truce condition -- opening access routes to Serb-besieged Gorazde in the east -- also could be difficult to achieve by Tuesday. Roads are neglected and strewn with mines.


UN spokesman Chris Gunnes said in Zagreb that opening a road to Gorazde from Sarajevo through Serb-held territory is a "very problematic task."


But the combatants deemed themselves committed to observing this latest truce of the 42-month-old war.


"I think this is a serious agreement, and it will be respected," Bosnian President Alija Izetbegovic said. "We will respect it, and I think the Bosnian Serbs will respect it."


Gunness said this truce had more chances of succeeding than the 36 others preceding it, because of "American involvement [and] the balance of power" between the warring sides.