An explosion ripped through a minibus traveling in a militant-infested area of northwestern Pakistan on Monday, killing 18 people, police said.

There were conflicting reports of whether the blast was caused by a bomb or by the gas cylinder used to power the vehicle. The bus was traveling between the cities of Hangu and Kohat, which are located 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, said Hangu police chief Abdur Rasheed. One person on the bus that tipped over was killed and 11 others were wounded, he said.

Rasheed said the blast occurred when the gas cylinder on board malfunctioned. But the top police official in the region said explosives were used to trigger the explosion.

Islamist militants frequently carry out attacks in the area against both civilians and security forces.
Local television footage showed the twisted carcass of the first bus laying 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 splattered across the outside.