Eighteen people were killed when an explosion ripped through a minibus travelling in a militant area of north-west Pakistan today, police said.

There were conflicting reports on whether the blast had been caused by a bomb or by a gas cylinder used to power the bus, which was travelling between the cities of Hangu and Kohat, close to Pakistan's lawless tribal region.

The explosion tore apart the vehicle, killing all 17 people on board, and tipped over a second bus nearby, the Hangu police chief, Abdur Rasheed, said. One person on the other bus was killed and 11 others wounded, he said.

Rasheed said the blast happened when the gas cylinder on board malfunctioned, but the leading police official in the region claimed explosives had been used to trigger the blast.

Islamist militants frequently carry out attacks against both civilians and security forces in the area.

Local television footage showed the twisted frame of the first bus lying beside the road, with little left except its wheels and undercarriage. The second bus was on its side, with its windows blown out and blood stains visible on the outside.