BELGRADE — The Yugoslav Army and Croatian forces blasted each other with artillery and mortars across Croatia yesterday in some of the fiercest battles of their four-month conflict.In heavy fighting since Friday, at least 33 people have been killed and more than 10,000 refugees driven from their homes. The clashes erupted despite the threat of European Community sanctions on any republic that does not accept an EC plan to transform Yugoslavia into a loose grouping of its six republics. Only Serbia has rejected the latest EC attempt to end the conflict prompted by Croatia's declaration of independence in June. Talks on the proposal are scheduled today in The Hague. The official Tanjug news agency said the Serb-led Army and Serbian irregulars had landed on the Croatian bank of the Danube river Sunday and started "final operations" to seize the town of Vukovar after a siege of 10 weeks. The commander of the vastly outnumbered Croatian militia defending Vukovar said the town could fall within hours unless they received urgent reinforcements. "Vukovar is in a very difficult and serious situation," the commander, Milan Dedakovic, said on the radio. Croatian media said MiG-21 jet fighters fired on Vukovar hospitals and other buildings. Air-raid sirens wailed in the capital, Zagreb, and in other towns in the center of the rebel republic where battles raged. In Belgrade, Serbia's ruling Socialist Party of former Communists met ahead of a session of parliament to consider whether to accept the EC plan, which would formally end the 73-year-old Yugoslav state in its present form. Serbian President Slobodan Milosevic was defiant. Tanjug quoted him as saying in talks on Sunday with Greek Foreign Minister Antonis Samaras: "Serbia cannot accept any ultimatums or the pressures it has been subjected to." The EC has brokered most of the 10 cease-fires that have collapsed since clashes began. Croatia says more than 2,500 Croats and an unknown number of Serbs and federal soldiers have been killed since June.