Pakistani officials say gunmen disguised in military uniforms have flagged down a convoy of commuter buses in a northern province and shot dead 18 Shi'ite passengers.

Police said the gunmen ambushed the four buses early Tuesday in the mountainous district of Kohistan. The attackers boarded the buses, checked the identity of the passengers, and opened fire on the Shi'ite passengers.

The Associated Press reported that the Pakistani Taliban, a Sunni Muslim extremist group, has claimed responsibility for the attack, which happened about 340 kilometers north of the capital, Islamabad.

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.