The bus, which was coming from Peshawar, reached near Pirak area of Sibi, and the driver parked it at a roadside hotel.

Four armed men riding on motorcycles reached there and two of them entered the bus. After sprinkling the vehicle with petrol, they put it on fire. At least 15 passengers were killed and many others suffered burn injuries.

Personnel of Levies and other law enforcement agencies rushed to the scene and took the bodies and injured to the Sibi civil hospital.

"The toll may go up as the condition of most of the injured is serious," sources said.

Police conducted search operations for arrest of attackers.