Pakistan authorities say unknown gunmen have attacked a bus carrying Shi'ite Muslim pilgrims in the southwestern part of the country, killing about 20 of them.

Officials in Pakistan's Baluchistan province said the attack occurred Tuesday as the bus was traveling to Iran.

While no one has claimed responsibility, Pakistan has a history of sectarian violence with attacks similar to Tuesday's. The country has a Sunni Muslim majority, and although most Sunnis and Shi'ites coexist peacefully, extremists often target members of each community.

In other violence, Taliban gunmen attacked a Pakistani military checkpoint in the country's northwestern tribal belt, leaving at least 19 people dead.

Officials said the militants launched the attack in the Dabori region of Orakzai tribal agency, killing at least one soldier and wounding five others.

Security forces were able to kill more than 18 militants in the ensuing gunbattle. Pakistani helicopter gunships also were involved in the fight.

There was no independent confirmation of the death toll. The area is largely shut off from journalists and aid workers.

Last year, the Pakistani military launched an operation against the Taliban in Orakzai. Pakistan's semi-autonomous and remote tribal region near the Afghan border has been the center of militant activity with domestic and Afghan Taliban elements located there as well as al-Qaida operatives and other foreign fighters.