Gunmen have attacked a bus carrying Shi'ite Muslims in southwest Pakistan, killing at least 13 people in the second such attack in two weeks.

Police say four attackers on motorcycles ambushed the bus early Tuesday on the outskirts of Quetta, the capital of Baluchistan province. Gunmen lined the passengers up and opened fire on them.

Authorities say 12 Shi'ite Muslims, mostly from the Hazara ethnic community, were killed. One Sunni Muslim was also killed and at least six people were wounded.

No one claimed responsibility for the latest attack on Shi'ite Muslims. On September 20, gunmen carrying rocket launchers and Kalashnikovs attacked a bus bound for Iran, killing 26 Shi'ite Muslim pilgrims near Quetta.

The area has seen a recent increase in attacks on the Shi'ite minority population by Sunni militants loyal to al-Qaida and the Taliban.

Following Tuesday's attack, a group of angry Shi'ite demonstrators staged a protest at the hospital where the victims had been taken, setting fire to the bus used to transport the wounded after it was emptied. Protesters also called for the Pakistani government to protect religious and ethnic minorities.

Pakistan has a Sunni Muslim majority, and although most Sunnis and Shi'ites coexist peacefully, extremists often target members of each community.

Last month, Amnesty International criticized the Pakistani government for failing to address a rising trend of sectarian violence. The rights group said that it had counted at least 15 attacks against Shi'ite minorities across Pakistan.