At least 36 people most of them school students  were killed when a bus crashed on the motorway in Chakawal southeast of Pakistan's capital Islamabad on Monday.

The bus, which was carrying pupils from a private school on their way back from an excursion, is understood to have come off the road and fallen into a ravine.

Speaking from the scene in Chakwal district, Punjab province, about 100km southeast of the capital, senior police officer Chaudhry Salim said at least 37 people were killed, including teachers and school students.

He said the cause of the accident, which also left more than 55 injured, was brake failure.

The dead bodies and six to eight dead bodies were sent to district hospital Chkawal and Fauji foundation hospital,' he said.

Around 15 injured are under treatment in Allied hospital.

Official from the traffic police confirmed the accident and said 33 people, including 26 students, were killed on the spot.

He said the driver, his helper and the wife of the principal of the school were among the dead.

According to the sources, the bus was over loaded and many students were seated on the roof top of the bus.

DCO Faisalabad while condoling the incident announced a holiday today. All education institutes, offices and markets would remain closed across the district.