ISLAMABAD, Pakistan — Gunmen opened fire on a bus in southwestern Pakistan on Tuesday, killing at least 13 Shiite Muslims and wounding seven, officials said.

The attack took place on the outskirts of Quetta, the capital of Baluchistan Province, and appeared to follow what is becoming a grim pattern in which Sunnis belonging to the extremist sectarian group Lashkar-e-Jhangvi in the region attack minority Shiites, who are mostly members of the Hazara ethnic group.

On Tuesday morning, attackers riding in a pickup truck intercepted the bus carrying mostly Shiite day laborers traveling to a market. The gunmen forced non-Hazara passengers to get off the bus, then opened fire on the people remaining inside, said Abdul Khaliq Hazara, chairman of the Hazara Democratic Party.