Police official Abbas Majeed Marwat says the Wednesday morning bombing happened in an army area of the city.
He says nearly 40 other people were wounded.
Marwat says an improvised explosive device was planted under a seat in the bus.
No one has claimed responsibility. Suspicion likely is to fall on the Pakistani Taliban and their allied militants, who have been waging a war on the state for more than a decade, killing tens of thousands of people.