Negligence, not arson, is the likely cause of the deadly forest fire burning out of control in the Carmel Heights near the northern Israeli city of Haifa, police said Saturday.

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The forest fire is seen from a road near Haifa, in northern Israel. ((Avishag Shar-Yeshuv/Associated Press))

A spokesman for Israel's police would not go into detail about the kind of negligence involved or explain why authorities were now leaning away from earlier suspicions of arson. Further investigation is needed, police said.

Police spokesman Micky Rosenfeld said two people were arrested as part of the investigation. Media reports said the the suspects were 16-year-old boys from a village bordering on the Carmel region.

By midday on Saturday, firefighters had still not brought the flames under control, Rosenfeld said. About 40 square kilometres of land, at least half of it national forest, has been scorched, said a spokesman for the Foreign Ministry.

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A firefighter walks among trees burned in a forest fire outside Kibbutz Beit Oren, near Haifa, on Friday. ((Nir Elias/Reuters) )

The blaze in the pine forest near Israel's third-largest city began Thursday. The death toll from the fire reached 41 on Friday.  Most of the victims were prison guards whose bus became engulfed in flames on Thursday. They had been on their way to help evacuate a prison near the town of Usfiya.

The flames have forced about 17,000 people to flee their homes in several villages, including the artists' colony of Ein Hod. Police said the blaze had destroyed homes overnight in Ein Hod, as well as in the agricultural community of Nir Etzion and the youth village Yemin Orde.

More than 20 international aircraft arrived at the Ramat David Israel Air Force base in northern Israel on Friday morning, carrying firefighters and supplies.

Israeli Prime Minister Benjamin Netanyahu has expressed his thanks for the help, which is coming from Britain, France, Greece, Turkey, Bulgaria, Jordan, Cyprus, Egypt, Russia, the United States and other countries.

"They're standing shoulder to shoulder with us on the ground and in the skies fighting this terrible fire," he said in a statement to Reuters TV on Friday. "We're deeply moved and deeply grateful."

The Palestinian Authority has also sent firefighting units to the disaster zone.

"The three units of the Palestinian civil defence were sent to assist in extinguishing the fires in the Carmel," President Mahmoud Abbas's office said in a statement.

With files from The Associated Press