A group of unknown militants stormed two Karachi-bound buses in Pakistan killing at least 21 and abducting several others.

The attack took place on Friday, 29 May night when the coaches were en route to Karachi from Quetta.

Dressed in security forces' uniform, the gunmen, numbering between 15 and 20, intercepted the buses in Mastung, 40kms from the Baluchistan capital Quetta.

After setting the buses ablaze, the militants attempted to identify the passengers before taking them away. The victims' bodies were recovered from nearby foothills while many are still missing.

Family members and relatives of the victims have staged protests outside government buildings in Baluchistan demanding swift action.

A security operation is under way to rescue the remaining passengers. Following a brief exchange of gunfire in remote regions, the security forces managed to free at least five people.

"The victims were not settlers nor from Shiite sect," Baluchistan home secretary Akbar Hussain Durrani said, labelling the attack a terrorist act. Authorities said the victims were ethnically Pastun and were travelling from Pishin to Karachi.

Local reports cited Baluchistan Home Minister Mir Sarfraz Bugti as blaming India's premier intelligence agency Research and Analysis Wing (Raw) for the attack. He alleged this was in retaliation for the increasing unrest in disputed areas of Kashmir.

A three-day mourning has been announced in Baluchistan. Pakistan Prime Minister Nawaz Sharif has ordered a thorough investigation into the incident.