Four assailants attacked the bus carrying more than 50 piligrims near Mastung town around 50 km (30 miles) from the provincial capital of Quetta.

“They opened fire on the bus from all four sides. Then they got into the bus and fired again,” a police official in Mastung said.

Three more people were killed when gunmen opened fire on an ambulance near Quetta as it headed to the attack site in Mastung.

Sunni Muslim militants loyal to Al-Qaeda and the Taleban have carried out high-profile attacks on members of Pakistan’s Shiite minority in the past.

These militants have stepped up attacks against Shiites in Baluchistan in recent months.

Ethnic Baluch militants have been waging a low-level insurgency in Baluchistan for years for more autonomy and greater control over natural resources of the region.

Officials say there is no evidence linking them with Islamist militants.

At least 10 Shiites Hazara were killed in a suicide bombing near Quetta on the Muslim festival of Eid on Aug. 31.

Pakistan has seen a surge in violence since Al-Qaeda chief Osama Bin Laden was killed by US special forces in a secret raid in a Pakistani town in May.

Militants have vowed revenge for Bin Laden’s death.