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.

Kohistan also borders Swat Valley, a once powerful Taliban stronghold that the Pakistani military targeted several years ago in an effort to root out militants.

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

Meanwhile, officials in the northwestern city of Peshawar say an unknown assailant shot dead two foreigners — one of them a woman — in a market section of the city. The circumstances of the shooting and the nationalities of the victims were not immediately clear.