ZAGREB, Croatia (AP) _ The Croatian army and rebel Serbs called another truce today _ the third in as many days _ and agreed to let tens of thousands of refugees leave the war zone under U.N. escort.

Unlike the previous cease-fires, however, this one appeared to be working, after a lag of several hours. The U.N.-brokered agreement took effect at 7 a.m. (1 a.m. EDT), and the United Nations said guns fell silent along the road into Bosnia in the early afternoon.

Many refugees had been trapped by fighting that raged along the borders they were trying to reach.

The agreement _ more specific than the previous truces _ called for the Serbs to turn over all heavy weapons to the United Nations and stop fighting in the area south of Zagreb where an unknown number of Serb soldiers were holding out against a five-day Croatian army offensive to retake rebel lands.

In exchange, Croatia would open a corridor for rebel fighters and Serb civilians from the area where they are trapped east into Serb-led Yugoslavia. The United Nations would provide fuel and escort for the refugee convoys, which could begin as early as today, said U.N. spokesman Yuri Shishaev.

``The cease-fire was concluded, and I hope that it will enable safe passage for civilians and military,'' said Yasushi Akashi, the top U.N. official for the former Yugoslavia.

Serb fighters began turning over heavy weapons to U.N. collection sites near Topusko, 40 miles south of Zagreb, said Maj. Rita LePage, a U.N. spokeswoman.

And while fighting raged further south on the road leading from Topusko into Bosnia, around the border town of Dvor, the shooting stopped in the early afternoon, U.N. officials said.

The Croatian army and the Muslim-led Bosnian army, which crossed into southern Croatia in support of its Croat allies, had been battling rebel Serbs, who held the town.

U.N. officials in the Bosnian capital of Sarajevo said the army had agreed to withdraw from southern Croatia. The army's command said Bosnian forces were turning over positions to Croat troops, but not weapons and artillery they captured from the Serbs.

Sporadic shelling, anti-aircraft and small arms fire rattled the front lines of Serb-held eastern Croatia late Tuesday and this morning, said U.N. spokeswoman Kirsten Haupt. But there were no significant troop movements and no sign that Yugoslav army troops, poised across the border in Serbia, had crossed the border.

Croatia's offensive, which began with an attack on the rebel Serb heartland at dawn Friday, has created the largest single wave of refugees since fighting broke out in the former Yugoslavia four years ago.

Red Cross and U.N. officials say about 90,000 refugees have made their way into Serb-held northern Bosnia, with another 50,000 in Croatia, trying to leave. Some estimates have put the exodus at 200,000 people.

Most of the refugees in Croatia were trapped near Topusko. About 5,000 refugees were in the town itself and another 30,000 civilians and soldiers were camped around the town.

Even as the truce raised hopes that they would be allowed to flee, the United Nations was investigating reports that Croatian jets bombed refugees as they entered Bosnia, as well as reports of ``monstrous, inhumane'' atrocities near Dvor.

Croatian and Bosnian troops had been reluctant to let the refugees leave Croatia because an unknown number of rebel Serb soldiers were in their midst. The Bosnian government feared those soldiers might join Bosnian Serb fighters.

Akashi, the U.N. special envoy, said he had reports of indiscriminate shooting of refugees. He did not elaborate.

The United Nations was investigating at least three instances of possible atrocities in Dvor over the last three days. Spokesman Phil Arnold described the reported atrocities as ``monstrous, inhumane and against all international standards.''

He refused to elaborate. But Akashi accused the Bosnian army of executing five civilians, including people who were crippled, elderly and mentally retarded.

Asked to respond to the allegations, Bosnian Foreign Minister Muhamed Sacirbey told Associated Press Television, ``Let them confirm it first.''

U.N. spokeswoman Maj. Rita Lepage said the United Nations was also investigating the possible bombing of civilians Tuesday near Dvor.

She said two MiG-21s dropped four bombs on unknown targets. Minutes earlier, two MiG-21s, laden with weapons, had been seen leaving Zagreb airport in the direction of Dvor.

Gen. Zvonimir Cervenko, the Croatian army commander, denied that Croatian jets were attacking civilians. However, he said, ``it was possible that some civilians were killed'' in attacks on other targets. He charged Serb forces had used civilians near Dvor as a cover while they withdrew heavy weapons.

Meanwhile, the Red Cross planned an airlift of food and medical supplies to Banja Luka in Serb-held northern Bosnia. It would be the first humanitarian airlift to Serb-held areas since war broke out in Bosnia three years ago.