Pakistani officials say gunmen have killed seven people after opening fire on a passenger bus in the northwestern part of the country.

Local officials in the Kurram tribal region said the attack took place Saturday. All of the attackers managed to escape.

The bus was carrying Sunni Muslims through a mostly Shi'ite area. While no one has claimed responsibility for the attack, it is common for Sunnis and Shi'ites to clash in the area.

The Pakistani military launched an offensive against Taliban militants in Kurram, one of the semi-autonomous tribal regions that border Afghanistan.