JERUSALEM, Feb 16 (Reuters) - At last five Palestinian schoolchildren and a teacher were killed on Thursday when their bus overturned and caught fire after a collision with a truck on a rain-slicked road in the occupied West Bank.

A witness, identified only as Tayseer, said on Israel Radio he saw the truck swerve head on into the bus. The fire started in the front ... and we helped the children from the back. I saw many of them die immediately, he said.

The bus had been carrying several dozen children, from a kindergarten and an elementary school, on a field trip from Shufat refugee camp near Jerusalem to a park near the West Bank city of Ramallah.

Doron Yisraeli, head of the Israeli police traffic unit in the area, said it had been raining and the truck had braked and skidded into the bus on the road just north of Jerusalem.

Human error, combined with the difficult conditions, led to lethal consequences, Yisraeli said.

Israeli and Palestinian emergency crews cooperated in rescue efforts at the scene, where the charred wreckage of the bus rested on its side against a stone wall on the shoulder of the road.

Dr Ahmad Bitawi, director of Ramallah Hospital in the West Bank, said 5 children and a teacher were pronounced dead at the facility, which also treated 54 people injured in the accident.

Yisraeli said Israeli and Palestinian police were cooperating in the investigation.

Palestinian President Mahmoud Abbas declared three days of mourning in the Palestinian territories. Israeli Prime Minister Benjamin Netanyahu, on a visit to Cyprus, issued a statement expressing sorrow at the deaths.

(Writing by Jeffrey Heller; Additional reporting by Jihan Abdalla and Maayan Lubell; Editing by Maayan Lubell)