ZAGREB, Yugoslavia (AP) _ As federal troops and Croatian militias tested a day-old cease-fire with gunbattles Monday, officials said neighboring Serbia was pressing for an agreement allowing it to incorporate key sections of Croatia.

News reports spoke of fierce fighting around the central Croatian towns of Nova Gradiska and Okucani, with intense tank and mortar shelling of Vukovar, a Croatian town on the Serbian border.

The Yugoslav news agency Tanjug said Monday night that the fighting seemed to be ebbing, except for sporadic clashes in the eastern region of Slavonia, where Serb insurgents have fought Croatian militias since the republic declared independence June 25.

''Croatia has shown its teeth, and a completely new relation of forces has been established,'' said Croatian President Franjo Tudjman's top adviser, Mario Nobilo, explaining why he thought the truce would hold.

''The only language that (Serbian President) Slobodan Milosevic can understand is the language of force, and we have now shown that will no longer be effective,'' Nobilo said.

The federal army may soon pull back from the fighting and settle into Serb- held parts of Croatia where the population ''recognizes it as its own,'' said Borisav Jovic, Serbia's man on the eight-member federal presidency.

A Western diplomat, who spoke on condition of anonymity, said Serbia seeks an urgent meeting among Yugoslav leaders to approve the partial army withdrawal.

He said Serbia would ask at the meeting that Croatia be permitted to secede from Yugoslavia, but without territories now held by the Serbs. That would achieve Milosevic's aim of expanding Serbian territory before agreeing to Croatian independence. The military is dominated by Serbian officers whom the Croats claim are loyal to Milosevic.

Although Croatian leaders have expressed willingness to give greater autonomy to Serb-dominated areas, they refuse to give up Croatian territory. There are also disagreements over which areas of Croatia would be considered ''Serb-dominated.''

Croatia's 4.75 million people include about 600,000 ethnic Serbs, many of whom live spread throughout the republic. Serb militants have taken up arms in some regions to set up their own government or join their areas with Serbia.

Tensions between Serbs and Croats, Yugoslavia's two largest ethnic groups, are fueled by memories of the massacre of hundreds of thousands of Serbs by the pro-Nazi government of Croatia during World War II and retaliatory killings by Serbs.

The truce signed by Tudjman and federal Defense Minister Veljko Kadijevic officially took effect at 3 p.m. Sunday. At least five previous cease-fires in Croatia have failed to end the fighting that has claimed more than 500 lives.

Zagreb radio, monitored earlier by the British Broadcasting Corp. in London, said Yugoslav army tanks were trying to break through positions Monday at the village of Dragalic near Nova Gradiska.

Belgrade radio said two federal soldiers were killed and two wounded in fighting around Nova Gradiska.

Tank-fired grenades and mortars bombarded the Croatian border town of Vukovar for 90 minutes, Associated Press correspondent Tony Smith reported.

The shelling came from the Serbian side of the Danube River, which separates the two republics. It was not clear who fired first, but Croatian media said 12 people were injured.

The Croatian coastal town of Sibenik also came under mortar attack, Croatian radio said. It said one person died and four were injured.

Elsewhere, the vice president of an ultranationalist Croatian party was shot dead by police. Dobroslav Paraga, head of the Croatian Party of Rights, charged that Ante Paradzik's death Saturday outside Zagreb was an assassination by the Croatian government.

Police said that the car in which Paradzik was riding refused to stop at a roadblock.

Paraga's party demands complete independence for Croatia and says the republic should not give up an inch of its territory to ethnic Serbs.