He said two buses carrying pilgrims were coming from the Iran border when one of them was hit by the explosion.
"We are still trying to ascertain whether it was a suicide attack or a roadside bomb but the explosives used were around 100 kg," Anwar said.
Two security vehicles escorting the buses were damaged in the blast.
No group claimed responsibility for the attack.
Earlier in the day, a noted Shia playwright was shot and injured by unidentified gunmen in the eastern city of Lahore.
Buses carrying Shia pilgrims have been repeatedly targeted in desolate areas of Balochistan in recent years. Scores have died in these attacks, many of them blamed on the banned Lashkar-e-Jhangvi.
