The incident took place in Dringhar area, on the main Quetta-Taftan Highway in Mastung district, some 50 kilometres southwest of Quetta.
"The bomb exploded after an FC (Paramilitary Frontier Corps) vehicle stopped to check a parked car," Sayed Mehrab Shah, a senior government official told Geo TV.
All the pilgrims on the bus, who were on their way to Iran, were safe but several security personnel were wounded by the remotely triggered bomb, Shah said.