A car bomb exploded near a university bus in Pakistan's insurgency-torn southwest, killing four people and injuring more than 40 others.

The attack took place on the outskirts Quetta, capital of Balochistan province, bordering Afghanistan and Iran.

Most of the victims in the blast were reported to be Shiite students from the Information Technology University in Quetta, who were passengers on the bus.

"An improvised-explosive device planted in a car parked on the roadside exploded near the bus of the local IT university, killing four people and wounding over 40 others, mostly students," police chief Mir Zubair said.

The injured were taken to a military hospital for treatment.

Balochistan has been riven by sectarian violence between separatist groups and Islamist insurgents linked to the global terror network al-Qaeda.

Pakistan's Sunni majority make up 80 percent of the population in the area, while Shiites represent the remaining 20 percent.

Although no group has claimed responsibility for the blast, police said it was likely to be a sectarian-led attack.

"The bomb targeted the bus as it carried a majority of Shiite students," Zubair said.