ZAGREB, Croatia (AP) _ Thousands of Croatian forces clashed with rebel Serbs today over a disputed highway in central Croatia, and three Jordanian peacekeepers were seriously wounded, U.N. officials reported.

The fighting came as a four-month truce unraveled in neighboring Bosnia. Skirmishes and explosions rocked the besieged Bosnian capital of Sarajevo today and rebel Serbs moved in upon U.N.-controlled weapons depots.

At least 2,500 Croatian troops advanced on both ends of a Serb-held section of the Zagreb-Belgrade highway some 75 miles southeast of Zagreb, said U.N. spokeswoman Susan Manuel.

Shortly after dawn, about 500 Croatian troops crossed into territory previously held by U.N. peacekeepers to keep the two sides apart, Manuel said.

When the Croats reached the Jordanian base near Novska, nearby Serbs apparently began shelling that wounded the peacekeepers, she said.

Croatian radio announced a general alert in several nearby towns today and warned of possible Serb terrorism.

Later, 2,000 Croatian soldiers carrying anti-tank weapons surged into Nova Gradiska, at the eastern end of the 16-mile strip of highway through Serb-held territory, Manuel said. Both Croats and Serbs reported shelling and shooting across the area.

Serbs also detained 26 U.N. policemen in the Serb-held town of Okucani, Manuel said.

The fighting came two days after five Croats and a Serb died in a clash over the highway, which was closed as a result. It opened last December for the first time since the Serb-Croat war of 1991, and was to have reopened today.

Tensions have soared in Croatia as the U.N. tries to scale back its mission there, compromising between demands from Croatia to leave and from Serbs to stay. The Serbs captured a third of the country in the 1991 war that killed 10,000 people.

Croatia described today's action as a limited police action to control traffic, while Serb military sources said the fighting erupted after Croatian army, not police, started an infantry and artillery attack on Serb positions.

In Bosnia, Serbs threatened two U.N.-controlled collection points containing heavy weapons around Sarajevo late Sunday, according to U.N. spokesman Maj. Guy Vinet. Firefights and explosions were recorded throughout Sarajevo overnight and today, he said.

Some 30 Bosnian Serb soldiers surrounded a Ukrainian-guarded weapons collection point in Serb-held Ilidza, a western suburb. Other Serbs occupied a French-guarded collection point further west in the suburb of Osijek, said another U.N. spokesman, Alexander Ivanko.

The peacekeepers, equipped with automatic rifles and 20-mm guns on their armored vehicles, prepared for battle.

The Serbs around the Ilidza depot withdrew today. But those in the other depot stayed on, preparing three cannon for what they said was a possible government offensive, Ivanko said.

Some 30 rocket-propelled grenades exploded along the front lines in the center of the city. Bosnian media reported increased movements of Serb troops and weaponry around Sarajevo today.

The U.N. said fighting picked up across Bosnia late Sunday, with shelling and explosions reported in Bihac in northwestern Bosnia, and Tuzla in the north.

Bosnia's civil war erupted three years ago when Serbs rejected the republic's decision to secede from Serb-dominated Yugoslavia. More than 200,000 people have been killed or are missing in the fighting.