YUGOSLAV leaders pleaded for peace after a crisis session to deal with spreading violence but found few concrete measures to salvage a crumbling cease-fire."The presidency demands that the cease-fire be established immediately," said the eight-man collective presidency late Saturday, acknowledging the truce they ordered 10 days ago had broken down. The presidents met after two days of clashes between Croatian security forces, Serbian guerrillas opposed to the republic's independence, and units of the federal Army in West Slavonia, a new conflict zone in Croatia. Several people were reported killed. Croatian Interior Minister Ivan Vekic said the Army fired across a river from the republic of Bosnia-Herzegovina into the town of Stara Gradiska, 75 miles east of Zagreb. Two Army planes then strafed evacuated police positions in the town. One dropped a bomb on the police station and destroyed it, Mr. Vekic said on Croatian television. He said there were no casualties. Vekic said Croatian forces blew up the town's bridge over the Sava river to prevent Army tanks massed on the other side from crossing into Croatia. The Army, which Croatia says is helping Serbian guerrillas to take over territory to be incorporated into a Greater Serbian state, confirmed the air attack but said the planes had first come under fire from the ground. Stara Gradiska is on the southern border of West Slavonia, a hitherto peaceful, ethnically mixed area. The Serb population there declared autonomy from Croatia last week. Fierce fighting erupted further north Saturday near the towns of Okucani and Daruvar and by the Belgrade-Zagreb motorway. The Aug. 7 cease-fire declared by the eight-man collective presidency dampened but failed to stop clashes which have killed more than 200 people in Croatia since it proclaimed independence June 25. The presidency, chaired by Croatia's Stipe Mesic, said in a statement that teams would immediately start monitoring the cease-fire in Croatia, whose government had approved the move. But a spokesman in the Croatian capital of Zagreb said earlier that the republic still objected to the ethnic makeup of the teams which it considered pro-Serbian. Diplomats said that, given the level of violence, the dispatch of Yugoslav observers now seemed irrelevant. The presidency agreed to send a group to Okucani, where at least two Croatian national guards and one Serbian guerrilla were reported killed in fighting since Friday.