At least 18 people have been killed after gunmen opened fire on a passenger bus in a northern Pakistan village in an apparently sectarian attack, police officials say.

The incident occurred on Tuesday in the remote mountainous village of Harban Nala in Kohistan district, about 200km north of the capital Islamabad, as the bus was carrying at least 25 passengers from Rawalpindi to Gilgit, police said.

"Armed men hiding on both sides of the road attacked the bus," local police chief Mohammad Ilyas said. "Initial reports said 18 people died and seven [were] wounded,"he said.

The motives of the attack were unclear, but a senior police official told Reuters news agency that all aboard the bus were Shia and that the gunmen belonged to a local Sunni group.

"It appears to be a sectarian attack," Abdul Sattar Khan, a local member of parliament, told the AFP news agency. "It is a remote area. We are getting reports that the gunmen forced people to get off the bus. They checked their papers and shot them dead."

Human rights groups have heavily criticised the Pakistani government for failing to curb sectarian violence between the country's majority Sunni and minority Shia Muslim communities that has killed thousands.

Sunni groups have targeted Shias with deadly attacks across Pakistan, particularly in the northwest.

Source: Agencies