Unidentified gunmen attacked a passenger bus travelling towards Karachi from West Pakistan.
The buses were en route to Karachi late on Friday when they were attacked in Mastung district, deputy district commissioner Akbar Harifal said.
Security forces rescued five passengers after an exchange of fire, he said.
No group has yet claimed responsibility for the attack.
Islamist militants regularly attack civilian and military targets; earlier this month at least 43 commuters were killed on a bus in Karachi by a group that has declared allegiance to Islamic State.
Since no group has yet claimed the responsibility of the attack it is quite difficult for the forces to take action.