Provincial police chief Chaudhry Mohammed Yaqoob said the bomb used "high-intensity" explosives and a timer. Witnesses said the back of the bus was destroyed, but the front was intact.

There was no immediate claim of responsibility for the bombing, which will deepen concerns over Baluchistan, where violence between tribal militants and security forces has escalated recently in remote areas of the province. The tribesmen resent the presence of security forces and demand more revenues for natural resources, including natural gas, extracted from their territory.

In other violence, a woman, her three children and a soldier were killed by rockets purportedly fired by tribal militants elsewhere in Baluchistan province. A land mine explosion killed another civilian. The bomb blast occurred in Kolpur, a town about 18 miles southeast of Quetta, the capital of Baluchistan, as the bus travelled with about 50 passengers from Quetta to the eastern city of Lahore.

CNN