"It was a terrorist attack," Hangu district police chief said, revising an earlier report that the powerful blast had been caused by exploding gas cylinder.
The police chief in Jozara near Hangu, a key city in Khyber-Pakhtunnkhwa province, said that suspected militants had planted 10 kgs explosives on board the bus.
"Preliminary reports by investigators and bomb disposal squads indicate that explosive materials fitted with a timer was placed near a gas cylinder, triggering the explosion," Hangu district police chief said.
All the 18 passengers on board the bus were killed and more people have been wounded critically in the high intensity explosion.
The minibus was heading to Kohat, another city located close to the restive tribal belt.
The injured were taken to a hospital in Hangu.
This was the third major incident in the area over the past one week. Two days earlier, heavily armed Taliban militants barged into the house of a police woman constable nearby, killing her along with five other family members.
On Thursday, in the adjoining Bannu district, a suicide bomber rammed his explosive-laden vehicle into a police station, killing 18 security personnel.