BELGRADE — The European Community offered to build up its peace observer force in Yugoslavia as four more people were killed there in civil strife but it said the country's feuding republics must first negotiate a cease-fire.More than 90 people have been killed in 10 days of ethnic violence in the republic of Croatia, where Croats and the large Serbian minority are fighting. The fresh EC offer to help prevent civil war in the Balkan country was made at a meeting between EC foreign ministers and Yugoslav Prime Minister Ante Markovic in Brussels Monday. "I told them openly that Yugoslavia is in a state of war - that it is facing an economic catastrophe and a social explosion," Markovic told reporters after returning to Belgrade. The Community also ordered a special three-minister mission back to Yugoslavia this week to assess the rapidly deteriorating situation. Dutch Foreign Minister Hans van den Broek said the EC made several offers, including increasing its peace observers to as many as 150 from 50 and extending their mandate to cover the areas where battles are raging between Serbs and Croats. He said Mr. Markovic and the Community ministers also agreed to create joint patrols along the line dividing Serbs and Croats. These would be made up of the Yugoslav Army and Croatian national guard units which fought against each other last week. But EC ministers emphasized their new offers were firmly conditional on Yugoslavia's warring factions hammering out a credible cease-fire, something which has proved impossible so far. The EC, seeking to raise its role in international affairs, negotiated a cease-fire in Slovenia after the Army stormed into the republic June 27.