A FRAGILE peace accord between federal authorities and the breakaway republics of Slovenia and Croatia has brought multiethnic Yugoslavia back from the brink of civil war.But escalating Serb-Croat strife in Croatia threatens to mar the agreement and deepen the federal Army's involvement in Yugoslavia's widening conflict. The tentative peace accord was reached yesterday after more than 16 hours of tense negotiations on the island of Brioni, off Croatia's coast, between Yugoslavia's eight-member collective presidency and Slovenian and Croatian leaders. European Community mediators also took part. The agreement formalizes a four-day truce in Slovenia that has held since lightly armed Slovenian forces repulsed Army tanks and war planes seeking to reimpose Yugoslav sovereignty. At least 62 people were killed in heavy fighting between federal troops and Slovenian defense forces for control of the republic's borders with Italy, Austria, and Hungary. Slovenia and Croatia declared independence June 25 after talks with federal officials over Yugoslavia's future structure broke down. The prosperous republics wanted to transform the nation into a loose confederation of sovereign states. Serbia, Yugoslavia's largest republic, and the pro-Serbian federal Army favor more centralized rule from Belgrade, the Serbian and Yugoslav capital. The new peace agreement suspends, but does not void, both independence declarations. In addition, the accord calls for a cease-fire throughout Yugoslavia and for a new round of talks on the Yugoslav crisis to begin next month. It sets a three-month cooling-off period on the Slovenian border issue. Slovenian police will continue to control the republic's 27 border posts, but turn over revenue from customs duties to the federal government. Control of Slovenia's border crossings, strong symbols of sovereignty to both sides, was the main issue in the talks on Brioni. The agreement also calls for federal Army units to return to barracks and for the deactivation of the Slovenian territorial defense force. Federal President Stipe Mesic, a Croat, declared that peace was at hand. But Dutch Foreign Minister Hans van den Broek, the leader of the European Community mission to Brioni, said the accord rested on "deep, dramatic complexities" and warned against overoptimism. "It remains to be seen whether we have had any long-term success." Croatian President Franjo Tudjman said, "Croatia has already had a little war going on for a year. I hope that the Brioni declaration will bring people to reason." Yugoslav officials said the joint declaration was accepted only "in principle" and that the Slovenian parliament, the federal government, and the federal presidency must still formally approve it. There was also speculation about whether the federal Army would abide by the accord. But Mr. Mesic, who as president acts as commander of the armed forces, said he expected the Army to go along with the agreement. The Army, which is dominated by pro-communist Serbian officers, has been accused of acting independently in the confrontation with Slovenia. However, Croatian Radio quoted Gen. Blagoje Adzic, chief of the Army general staff, as saying he could not accept the terms of the agreement. It was not clear whether General Adzic was speaking on his own or for the entire Army high command. Although the fighting has stopped in Slovenia, bloody clashes between Serbian nationalists and Croatian guardsmen in eastern Croatia have been raging for several days. Fighting between Croats and Serbs holds the greatest potential for reigniting widespread conflict. The new clashes have prompted the Army to intervene to keep the two sides apart. The heaviest fighting has occurred in and around the eastern Croatian village of Tenja, near the Serbian border, where increased numbers of federal troops and tanks have been deployed in recent weeks. Federal Defense Minister Veljko Kadijevic has appealed to both sides to end the fighting. "The most important thing is to achieve a political solution as soon as possible and to transform Yugoslavia without shedding new blood," he said. But he vowed the Army would continue to act to prevent further ethnic conflicts in Croatia. Federal forces in Croatia previously had acted only as a buffer between increasingly militant Serbs defying Croatian authority and special Croatian forces determined to reestablish it. More than 50 people have been reported killed in Serb-Croat clashes in Croatia since May. In broadcasts monitored in Zagreb, the Croatian capital, Belgrade Radio said the Army became embroiled in clashes in Tenja after the troops tried to set up a buffer zone. It said Croatian militiamen began firing on Army armored personnel carriers, and cannon on both sides then opened up, along with machine guns and mortars. Croatian television showed film footage of intense street fighting in Tenja. House walls were pocked by artillery shells, rocket-propelled grenades, and bullets. Windows were shuttered or shattered. "This is Lebanon," Croatian militaman Vladimir Flatscher told Croatian television.