An explosion near a university bus has killed at least three students and injured another 40 people, mostly students, in southwestern Pakistan, according to officials.

Monday's motorcycle bomb occurred in the upscale Jinnah Town neighbourhood on the outskirts of Quetta, the capital of the oil and gas rich Balochistan province that borders Afghanistan and Iran.

"An improvised-explosive device planted underneath a car parked on the roadside exploded near the bus of the local IT university", said Muhammad Anwar, a local police official.

The casualties were confirmed by another local police official, Akbar Shawani, who said the bus was transporting the students to the university at the time of the attack. Four policemen were among the wounded.

So far no group has claimed responsibility for the attack. Balochistan has been the site of sectarian violence targeting the Hazara Shia community and a separatist movement among Baloch groups demanding political autonomy and a greater share of profits from oil, gas and mineral resources in the region.

Human rights activists have accused the military of mass arrests and extra-judicial executions in its bid to quell the separatist movement.

Earlier this month, Navi Pillay, the UN human rights chief, voiced concern over allegations of "very grave" rights violations during Pakistani military operations.

Source: Agencies