At least 16 people have been killed in the bloodiest clashes between Serbs and Croats in Yugoslavia since World War II, increasing fears of civil war, Croatian officials said Friday.

The local council in the northern Croatian town of Vukovar said 12 Croatian policemen and three civilians were killed in several hours of shooting with Serbs Thursday in the nearby village of Borovo Selo.Borovo Selo was sealed off by the army Friday. There were no reports of further clashes.

Local hospitals said 14 police officers and six civilians were hurt in fighting before federal army tanks and soldiers intervened. Another policeman was killed in a gunfight in Palaca village in southern Croatia.

Croatian President Franjo Tudjman said war had begun after months of tension between Serbs and Croats, Yugoslavia's biggest nationalities and old rivals.

He accused Serbia of plotting the clashes and said he would have to reconsider relations with the rival republic. He appealed for calm but said he would mobilize civilians in trouble spots and use all means necessary to defend Croatia.

"We have experienced the most tragic day in the short history of the new democratic authorities, a year after the victory of young democracy in Croatia," Tudjman said.

"We are facing, I may say, the beginning of open warfare against the republic of Croatia."

Relations between Serbia and Croatia have deteriorated since the six republics in Yugoslavia held free elections last year that aggravated ethnic rivalries. Clashes have become more frequent this year.