Authorities in southwestern Pakistan say a bomb has exploded near a university bus, killing four students and wounding more than 40 people.

The blast happened near Quetta, capital of the restive Baluchistan province bordering Afghanistan and Iran.

Baluchistan is home to various militant groups, including pro-Taliban factions, and Baluch separatists fighting a low-level insurgency in hopes of gaining political autonomy and a greater share of the profits from the region's natural resources.

Amnesty International says Baluchistan is one of the most militarized regions of Pakistan, where armed sectarian groups, government security forces and criminal gangs operate with impunity.