JERUSALEM — Dozens of Israeli guards trying to rescue prisoners threatened by the worst forest fire in the country's history died Thursday when their bus became trapped in the same inferno.

As the guards raced toward the prison holding mostly Palestinians, a lone tree fell across the road, blocking their path. With no way out, many of them were burned alive inside the vehicle. Others perished while trying to flee the flames fed by brush left tinder-dry by lack of rain.

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When the smoke cleared, at least 36 were dead.

"This is a disaster of unprecedented proportions," Prime Minister Benjamin Netanyahu said.

The flames forced 12,000 people from their homes, leveled a village and threatened to cause irreparable harm to one of Israel's few forested areas. The fire was still burning out of control near midnight.

Israel issued a rare call for international assistance, a measure of the severity of the disaster. Turkey put aside recent tensions to pledge aid, and Netanyahu's office said Greece, Spain and Cyprus agreed to send firefighting helicopters. Additional aid was coming from Britain, Russia, Egypt, Azerbaijan, Romania, Jordan and Bulgaria.

Fire officials said the blaze had blackened some 1,600 acres (650 hectares).

Police also evacuated a university, three prisons and a hospital.

Investigators speculated that the fire could have been sparked accidentally, or it might have been deliberately set. But they largely ruled out any sort of attack by a Palestinian group.

The fire broke out around midday and quickly spread, fanned by unusually hot and dry conditions. Israel experienced an exceptionally warm summer and has had little rain during the normally wet autumn.

Flames ripped through the Carmel forest in Israel's Galilee region, eventually reaching the coastal city of Haifa after jumping from place to place in the forest.

Fourteen bodies were found near the charred skeleton of the bus 10 hours after the blaze began.

Netanyahu said the government was using all means at its disposal to contain the blaze, and he appealed for help from abroad.

Israel's appeal was a rare call for international assistance. The Jewish state is better known for sending its own rescue teams and medical personnel to other countries to help in their disaster-relief efforts.

Once close allies, Israel and Turkey have been in a crisis since Israel's bloody attack May 31 on a Turkish flotilla trying to break Israel's blockade on Gaza.

After nightfall, Netanyahu flew over the fire to inspect the damage. Speaking at the firefighters' command post, he said the blaze was of "international proportions."

He said the arrival of equipment from abroad on Friday could be decisive, but crews could not resume work until daybreak.

Netanyahu called a special Cabinet meeting for Friday morning to assess the situation.

Flames heavily damaged one of Israel's few large forests, made up of natural growth and planted areas, a favorite spot for camping, hiking and picnics. The woods provided a refuge for dozens of species of wildlife. Forestry workers tried to evacuate animals from the burning woods.

The forest recovered slowly after a fire in 1989, but experts said Thursday's blaze was many times worse.

Defense Minister Ehud Barak ordered the military to make all its resources available to fight the fire and rescue victims.

The military said it sent soldiers and equipment, including helicopters, bulldozers, medics and army units.

After sundown, evacuation orders were issued for several communities, as well as a neighborhood of Haifa, Israel's third-largest city, and a third prison. Haifa University, at the edge of the stricken Carmel nature preserve, was evacuated, too.

Kibbutz Bet Oren, a collective village in the wooded area, burned to the ground after its residents were evacuated, witnesses said.

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The military emptied one of its prisons near the fire area. A psychiatric hospital was evacuated, and a nature resort in the middle of the forest sent all its guests home.

The men aboard the bus were prison workers brought in as reinforcements from central Israel to assist in the rescue, said Yaron Zamir, a spokesman for the national prison service.

He called it a "difficult, sad and incomprehensible day."

Police said the commander of the Haifa police station, who was driving behind the bus, was critically burned.

Eli Bin, a spokesman for the Magen David Adom rescue service, said three bus passengers were evacuated to hospitals, two in serious condition and one with minor injuries.

Israel's president, Shimon Peres, expressed sorrow for the loss of life and praised firefighters trying to contain the blaze.

"They exemplify personal and superior bravery, and we are praying for a miracle," Peres said in a ceremony marking the Hanukkah holiday. "We pray for their safety. We pray for the cessation of the fire."

Peres' office said he later spoke to the Palestinian prime minister, Salam Fayyad, who offered condolences.