A bomb ripped through a passenger bus in a province of southwestern Pakistan wracked by growing tribal unrest, killing at least 13 people Sunday and wounding 20 others, police said. Some people were still trapped in the wreckage.

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 traveled with about 50 passengers from Quetta to the eastern city of Lahore.

Provincial police chief Chaudhry Mohammed Yaqoob said 13 people were confirmed dead and 20 others injured, but he said an uncertain number of bodies were trapped inside the twisted wreckage and could only be removed by steel cutters.

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.

Interior Minister Aftab Khan Sherpao told state-run Pakistan Television it was too soon to say who was behind the bombing, but Yaqoob suggested it was linked to tribal violence.

"This is part of the situation that has been going on in Baluchistan for the past quite some time, and those people have a hand in this who have been involved in this type of explosion," Yaqoob told reporters.

A survivor said the explosion happened while the bus was moving.

"It was a fearsome and very large explosion," Lal Mohammed, 34, who suffered injuries to one of his legs and chest, told The Associated Press.

"There was blood in the bus. I was hit by somebody's severed hand and I lost consciousness," he said from a bed in the state-run Civil Hospital in Quetta.

Deadly rocket attacks on troops and state-run facilities are common in the remote deserts of Baluchistan, as are small bombings in and around Quetta. But if Sunday's blast is linked to the tribal unrest, it would be a serious escalation of the conflict.

On Saturday, militants fired heavy barrages of rockets at security forces and natural gas installations, killing a soldier and two civilians. They also blew up a gas pipeline at Dera Bugti, about 180 miles east of Quetta.

The militants continued firing rockets on Dera Bugti and a nearby gas field Sunday, killing a woman, her three children and a soldier, said Pir Bakhsh, a police official in Dera Bugti. The four family members died when three rockets struck their home, he said.

Authorities blamed fighters from the Bugti tribe, whose leader is an outspoken advocate for the rights of ethnic Baluch people native to the province -- Pakistan's poorest but the location of its biggest gas fields.

The continued unrest in Baluchistan's tribal areas has raised fears of a repeat of violence that rocked the province in the 1970s, when thousands died in a large-scale military operation against rebellious tribesmen.

On Friday, President Gen. Pervez Musharraf denied the army was mounting a military operation in Baluchistan but demanded that local militia disarm. He vowed to establish the government's authority in the province.

Elsewhere in Baluchistan, a pickup truck hit a land mine along a dirt road Sunday, killing the driver of a civilian vehicle in the tribal area of Kahan, said Rasool Bakhsh, a local police official.