European Community foreign ministers trying to avert civil war in Yugoslavia prepared to seek government support yesterday for deploying EC cease-fire observers in the rebel republic of Croatia.The Croatian parliament accepted a call by the Yugoslav state presidency Saturday for an immediate halt to fighting in Croatia, where hundreds of people have died during weeks of ethnic strife. The presidency ordered Croatian forces, the Yugoslav Army, and Serbian guerrillas to withdraw from each other's range. Ministers from the Netherlands, Portugal, and Luxembourg were scheduled to meet Yugoslav Prime Minister Ante Markovic in Belgrade after talks with the presidents of the Serbian, Croatian, and Slovenian republics. The ministers are carrying a memorandum that sets out the terms under which observers would be sent to Croatia. About 300 people have been killed in fighting since Croatia and Slovenia declared their independence from Yugoslavia on June 25. The Serbian minority living in Croatia is resisting the republic's secession.