Pakistani police say gunmen killed at least 12 people in an attack on a bus in the restive southwestern city of Quetta on Tuesday.

Police say the unidentified gunmen 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.

Pakistan has seen a series of sectarian attacks targeting Pakistan's Shi'ite minority. The country has a Sunni Muslim majority, and although most Sunnis and Shi'ites coexist peacefully, extremists often target members of each community.