The bomb was planted beneath a passenger seat and exploded in the outskirts of Peshawar, while the bus was en route to the town of Hangu.
"At least seven people have been killed and six wounded, the bomb was planted under a seat," senior police official Najeebur Rehman told AFP.
Syed Jameel Shah, a spokesman for Peshawar's main Lady Reading Hospital, confirmed the death toll.

PHOTO: MUHAMMAD IQBAL
Bomb Disposal Unit's (BDU) Shafqat Malik said the bomb contained five kilograms of explosives and was planted under a passenger seat.
Three bodies have been identified, while the remaining four are charred beyond recognition.
Further investigations are underway.
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