A bomb has ripped through a passenger bus in southwestern Pakistan, killing at least 13 people and wounding 20 others.

The police chief of the Baluchistan province, Chaudhry Mohammed Yaqoob, says the toll could rise because some people were still trapped inside the wreckage.

Officials say the bus was carrying about 50 passengers from the provincial capital, Quetta, to the eastern city of Lahore when the bomb went off Sunday.

No one has claimed responsibility. But the police chief blamed Baluch tribal militants who have stepped up their insurgency seeking greater autonomy and more compensation for the region's gas and other natural resources.

The bus bombing came a day after a rocket attack blamed on Baluch tribal militants killed a soldier and wounded at least two people. Tribal militants also blew up a gas pipeline and fired more than 200 rockets at the main paramilitary base in the region.

Some information for this report was provided by AFP, AP and Reuters.