At least 10 others were wounded by the blast on the minibus in Hangu district, Khyber Pakhtunkhwa province.

A bomb-disposal squad said the explosion had been caused by a remote-controlled device.

Police had earlier not ruled out the blast being an accident caused by fuel containers on the vehicle.

Two exploded gas cylinders were spotted in the wreckage of the bus, which had been carrying 18 passengers from Hangu to nearby Kohat.

Police also revised down the death toll from 15 to 11.

It is not clear why militants would want to attack a bus that was thought to be carrying civilians.