ISLAMABAD - At least 18 people were killed and eight others injured when a passenger bus was attacked by unknown gunmen early Tuesday morning in northern Pakistan, reported local media.

Local state-run Urdu TV channel PTV quoted officials in the district as saying the attack took place at a pre-dawn time in the country's northern district of Kohistan, a mountainous area lying some 160 kilometers north of the country's capital Islamabad.

Eight gunmen dressed in army uniform stopped a passenger bus on the Karakoram Highway in the Harban area of Kohistan. They forced all the passengers on board to get out at their gunpoint and started shooting at them indiscriminately.

The bus was then on the way from Rawalpindi, a sister city of Islamabad, to Gilgit, capital of Gilgit-Baltistan province in northern Pakistan, which borders China. Local media said that there were altogether 26 people on board when the bus was attacked, but some reports put the figure at 39.

All the injured people have been shifted to hospitals in Kohistan and Gilgit. Hospital sources said that seven people were critically injured.

The motive behind the attack is not known yet. No group has claimed responsibility for the attack.

Local watchers believe that it could be a sectarian attack as many of the residents in Gilgit and its surrounding areas are minority Shia Muslims. They are in constant clashes with the Sunny Muslims.

Earlier this month a Sunny Muslim leader was killed in Gilgit. Tuesday morning's attack in Kohistan could be a retaliatory action taken by the Sunny Muslims over the killing of their leader.