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 at least 12 Shi'ite Muslims, mostly from the ethnic Hazara 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 Sunni militant group Lashkar-e-Jhangvi said it carried out that ambush.

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.

Amnesty International on Tuesday said that sectarian violence has been a feature of the “general breakdown in law and order” in Pakistan, but that these recent attacks indicate a new targeting of the Hazara community.

Amnesty's Pakistan researcher said routine targeted killings against the Hazara and other groups raise serious questions about the “will or ability” of Pakistan's security forces to protect the people of Baluchistan.

Elsewhere, eight Pakistanis working for a U.S. charity have reportedly been released after being kidnapped near Quetta in July.

The French News Agency (AFP) and the Pakistani newspaper Express Tribune said the American Refugee Committee workers were freed on Tuesday in Pakistan's northwest, after being taken to the South Waziristan tribal region by their abductors.

It was unclear who kidnapped the aid workers and whether a ransom was paid.

The Pakistanis were working at an Afghan refugee camp in Baluchistan.