SARAJEVO -- Peace prospects in former Yugoslavia were jolted Tuesday when Bosnian troops bombarded Serb forces around Sarajevo, but there were tentative steps toward resolution of a dispute between Croatia and Serbia over the region of eastern Slavonia.


The Bosnian army fired four heavy weapons from inside the 20 kilometer exclusion zone around Sarajevo at Bosnian Serb positions, drawing a harsh rebuke from UN peacekeepers.


"The United Nations is aware of four Bosnian government heavy weapons inside the exclusion zone currently firing at the Bosnian Serb Army," UN spokesman Alexander Ivanko said.


"This is an outrageous action and we will be protesting to the Bosnian government at the highest level," he added.


Bosnian Serbs were forced to pull their heavy weapons back outside the exclusion zone in September in exchange for a halt to NATO air strikes against them.


The Moslem-led Bosnian government had agreed not to use its own heavy weapons to attack the Serbs from inside the zone.


"Now we will see the Serbs demanding to be allowed to bring their heavy weapons back inside the exclusion zone to attack government gun positions," said a UN official who asked not to be named.


In Croatia, the Zagreb government and Serb insurgents occupying its Eastern Slavonia region agreed on a preliminary set of "guiding principles" aimed at a settlement of their dispute, UN envoy Thorvald Stoltenberg said.


"These basic principles represent an important first step to peacefully resolve the question," he said in the town of Erdut, located on the Croat-Serbian border.


Stoltenberg and Peter Galbraith, the U.S. ambassador to Croatia, mediated at the talks in Erdut between Croatian envoys and leaders of local Serbs who have held the area since 1991.