Gunmen wearing military uniforms stopped a convoy of passenger buses in the northern Pakistani district of Kohistan Tuesday and shot dead 18 Shi'ites.

Local authorities said the buses were traveling through the mountainous district to the northern town of Gilgit at the time of the ambush. They said the attackers boarded the bus, checked the identity of those on board and opened fire on the Shi'ite passengers.

Local media quoted representatives from the banned Sunni militant group Jundullah as claiming responsibility for the attack. The Associated Press reported that the Pakistani Taliban, another Sunni Muslim extremist group, also claimed responsibility for the attack.

U.N. Secretary-General Ban Ki-moon strongly condemned the killings. Mr. Ban said the U.N. stands by Pakistan in its efforts to combat terrorism and extremism — which he said “continue to claim the lives of so many Pakistanis.”

In other violence, an unknown assailant shot dead a Chinese woman and her male Pakistani companion, while they were walking in the northwestern city of Peshawar.

The circumstances of the shooting were not immediately clear, but according to an eyewitness, the two were taking pictures of a nearby church.

Kohistan 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.