At least fifteen people were killed when a bomb exploded on a bus carrying government officials in the northwest Pakistani frontier city of Peshawar on Wednesday, officials said.

The attack took place on a main road after the bus picked up government employees from areas surrounding Peshawar and was transporting them to work in the city.

"It's premature to comment about the nature of the blast but it appears that explosives were planted inside the bus," senior police official Mohammad Kashif said. "There were 40-50 people on the bus."