Police said that some of the victims appeared to have been civilians travelling in other vehicles which were near the bus when the blast took place.
Officials said the bomb appeared to have been detonated by remote control, but said the type of bomb was still being investigated.
Investigators were seen gathering evidence and material from the site. Two damaged motorcycles and bicycles lay at the scene, along with scattered debris, an AFP news agency photographer said.
The attack on a busy road between Peshawar and Kohat came at a time of heavy fighting between Pakistani forces and pro-Taliban fighters in the tribal areas near the border with Afghanistan.
More than 150 people, most of them fighters, have reportedly died in clashes in the Bajaur tribal area over the past week.