Pakistani officials say gunmen have opened fire on a passenger bus in a northern province, killing at least 18 people.

Police say the bus was travelling in the mountainous district of Kohistan early Tuesday when it was ambushed by a group of armed men.

Officials say the attack appeared to be sectarian. They say the victims were Shi'ite Muslims and that the attackers may have been members of the local Sunni community.

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.