Gunmen have shot and killed five Shi'ite Muslims and a United Nations employee in two separate attacks in southwest Pakistan.

Police say assailants on motorcycles opened fire on a passenger van in an apparent sectarian attack in Quetta, the capital of Baluchistan province, on Thursday.

A woman was among those killed and at least six people were wounded in the attack.

The killing sparked protests in parts of Quetta, with demonstrators setting motorcycles on fire.

Pakistan has seen sectarian attacks targeting the country's Shi'ite minority, which makes up about 15 percent of the population. Pakistan has a Sunni Muslim majority, and although most Sunnis and Shi'ites coexist peacefully, members of both communities are often targeted by extremists.

In a second attack Thursday, officials say gunmen opened fire on a vehicle in the Mastung district of Baluchistan, killing a United Nations Food and Agriculture Organization worker and the driver.

There was no immediate claim of responsibility for the killing.

And violence continued Thursday in Pakistan's southern port city of Karachi, where at least four people were killed overnight.

The unrest in Sindh's provincial capital began earlier this week when two political activists were shot dead. Businesses have been shut down and security forces dispatched to the country's economic hub to help maintain order.