Fifteen people were wounded in the blast near the town of Kohat.

"It is not yet confirmed if it's a suicide attack or the bomb was planted on the bus," senior top police official Shahidullah told Reuters by telephone.

Militants have carried out numerous suicide bombings and other attacks on soldiers, security forces and police in a bid to destabilise the US-backed government.

The government has said military offensives have weakened the Taliban -- who are linked to al Qaeda -- but militants often move around and establish new strongholds after being dislodged.