ZAGREB, Croatia (AP) _ Croatian defense officials accused rebel Serbs of trying to provoke a new war after the Serbs reportedly shelled a vital bridge linking Croatia's Dalmatian coast to the rest of the country.

The dispute over the Maslenica bridge is heightening tensions between the Serbs and Croats in Croatia, who fought a six-month war in 1991 that killed 10,000 people.

Serbs fired nine shells Thursday at the bridge east of the Adriatic port of Zadar, said AP photographer Hrvoje Knez, who was there. None of the shells hit the bridge and no one was injured.

Croatian Serbs denied they shelled the bridge and accused the Croats of ''stage-managing'' attacks.

But the Croatian Defense and National Security Council, headed by President Franjo Tudjman, charged in a statement late Thursday that the new shelling of the bridge was to ''provoke a war conflict in Croatia,'' Croatian radio reported.

Serbs captured the bridge and surrounding area during the 1991 war, but Croatia later took it back. Last weekend, Croats were to hand over the bridge to U.N. officials under an accord worked out between the Serbs and Croats, but they refused.

Serbs partially sank the bridge Monday, and Tudjman said that the shelling had changed circumstances and that his troops would not withdraw.

Col. Kosta Novakovic, deputy commander of the Croatian Serb forces, denied his troops shelled the bridge Thursday, the Yugoslav news agency Tanjug reported. He said the detonations were the Croats destroying Serb houses in the area.

U.N. spokesman Simo Vaatainen in Zagreb said about 20 reporters were at the bridge when the shelling started, watching Croatian army divers and a U.N. team inspect the bridge for damage. U.N. observers were moved to safer areas, he said.

Knez, whose car took a direct hit, said the shelling appeared to be directed at people around the bridge.

With parts of all other routes to southern Croatia under Serb control, the span is part of the only land route from the coast to the rest of the country. Otherwise vehicles must be shuttled by ferries.

Fighting also was reported elsewhere in Croatia.

Croatian media reported Serbs fired about 400 artillery rounds at Gospic, 90 miles southwest of Zagreb. Three people were injured in the attack. Croatian army positions in Josipdol and Ogulin, about halfway between Zagreb and Gospic, also came under fire, Croatian radio said.

Gunmen ambushed a Croatian police patrol in a U.N.-controlled zone east of Zagreb, killing four of them and wounding four others, officials said today.

Mladen Lackovic, a Croatian police spokesman, said the patrol was investigating several recent explosions near Pakrac in central Croatia when they were attacked Thursday.

Two gunmen appeared in front of their truck with a machine gun and opened fire without warning, he said.