The blast occurred in a car parked at a petrol pump near the Quetta-Karachi Highway, Geo TV said citing police officials.
Four people were killed on the spot and three others injured in the blast, some three kilometers from the Sorab area, the police said. The injured were rushed to a hospital.
The police have cordoned off the area and has started investigation. Eyewitnesses said it was a huge blast that damaged nearby buildings.