Two men were killed in the rebel republic of Croatia in mortar attacks Saturday night that violated a cease-fire in Yugoslavia's ethnic fighting.Croatian radio said a villager was killed in an attack on the village of Hrastovica, southeast of Zagreb. It blamed the attack on "terrorists a reference to Serbian guerrillas who fiercely oppose Croatia's bid to secede from Yugoslavia. The second victim, a Croatian television cameraman, died following an attack in the nearby town of Kostajnica, Croatian television said. The clashes were the latest in a series that have undermined a six-day-old truce between warring Serbs and Croats. The violence and mutual distrust between Serbian and Croatian officials have delayed plans to exchange prisoners taken during weeks of fierce fighting in which at least 300 people were killed. A first exchange of prisoners, brokered by a cease-fire monitoring commission set up by the Yugoslav presidency, took place amid tight security Saturday night at a federal Army barracks in the Croatian town of Vinkovci, 100 miles northwest of Belgrade. Two Croatian national guardsmen were exchanged for three Serbian reservists. Croatian officials said they hoped the swap heralded the start of a peaceful solution to the crisis. But Serbian officials boycotted an earlier prisoner exchange in the eastern Croatian town of Osijek, saying they feared arrest by Croatian security forces. They called for an alternative meeting place. Croatia, whose lightly-trained national guard has been battered by Serbian guerrillas, is believed to hold about 60 Serbs. The Serbs, who the Croats say are backed by the federal Army, are said to have twice the number of Croats.