Up to 40 people are dead after a bus fleeing a massive forest fire in northern Israel overturned and went up in flames, killing dozens of prison guards.

Fire officials said the fire covered 325 hectares Thursday night and was one of the deadliest accidents in Israeli history. As nightfall arrived, the fire still raged out of control.

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

The guards were helping rescue inmates from a prison when their bus went up in flames. Television footage showed the charred metal remains of the vehicle, with dozens of bodies on the ground.

Yoram Levy, speaking for the national fire service, said 40 people have died as a result of the fire, mostly in the bus accident.

Two dozen more are feared missing.

Officials suspect the fire started from someone burning garbage before midday. Israel said Greece and Cyprus have offered to help fight the blaze.

Eli Bin, a spokesman for the Mada rescue, said only three people survived the bus crash and all suffered severe burns.

Israel had an extremely hot summer and has had little rain of late, allowing the fire to spread easily.

"There is a fire burning, and we are not in control of these areas," Yona Yahav, mayor of the northern city of Haifa, said. "It's a sad day for the country."