At least 30 people -- most of them school students -- were killed when a bus crashed on the motorway southeast of Pakistan's capital Islamabad on Monday.

At least 30 people -- most of them school students -- were killed when a bus crashed on the motorway 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 30 people were killed, including 24 school students.

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

'We have received 24 dead bodies and six to eight dead bodies were sent to another nearby hospital,' he said.

Another official from the traffic police confirmed the accident and said 26 people, including 22 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.