At least twelve people have been killed in northwest Pakistan as a suicide bomber attacked a convoy of Shia Muslims guarded by security forces, police have said.

The victims were passing through a petrol station in the town of Hangu on Friday when the lone attacker on foot set off the bomb, Akram Ullah, a police official, said.

Thirty civilians were injured in the blast. No casualties were reported among the security forces escorting the buses.

"Our convoy was hit by a big explosion," Javed Hussain, who was in the bus convoy travelling to the city of Peshawar, said.

"It's all chaos here. I myself have seen four dead, two of them are children. I have seen four wounded women."

Tensions between Pakistan's majority Sunni Muslims and Shias have made the road unsafe for minorities travelling to the nearby Kurram tribal region.

Police recently had declared it safe, but Shias are provided security to travel through it.

More than 4,000 people have died in sectarian violence between the country's Sunni majority and Shia minority since the late 1980s.