Pakistani police say gunmen killed at least 12 people Tuesday during an apparent sectarian attack on a bus carrying Shi'ite Muslims near the restive southwestern city of Quetta.

Police say unidentified gunmen on motorcycles ambushed the bus, lined up the passengers and opened fire on them. At least six people were wounded in the attack.

Authorites say at least nine of those killed were Shi'ite Muslims, mostly from the Hazara ethnic community.

The attack happened on the outskirts of Quetta, the capital of Pakistan's oil-rich southwestern Baluchistan province, which borders Iran and Afghanistan.

Sunni militants loyal to al-Qaida and the Taliban have recently increased attacks on the Shi'ite minority population in the area.

Last month near Quetta, gunmen carrying rocket launchers and Kalashnikovs attacked a bus bound for Iran, killing 26 Shi'ite Muslim pilgrims.

Pakistan has a Sunni Muslim majority, and although most Sunnis and Shi'ites coexist peacefully, extremists often target members of each community.