An ethnic Hazara Shi'ite man is comforted by his relative after he arrived at the local hospital in Quetta, to find a family member shot dead, Sept 20, 2011. Gunmen opened fire on a bus in Pakistan's southwestern province of Baluchistan in a suspected sectarian attack on Tuesday, killing at least 26 Shi'ite Muslim pilgrims travelling to Iran, police said. [Photo/Agencies]

QUETTA, Pakistan - Gunmen opened fire on a bus in Pakistan's southwestern province of Baluchistan in a suspected sectarian attack on Tuesday, killing at least 26 Shi'ite Muslim pilgrims travelling to Iran, police said.

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-Qaida and the Taliban have carried out high-profile attacks on members of Pakistan's Shi'ite minority in the past.

"Two vehicles intercepted the bus. Forced all the passengers off and opened fire. Many of them fled," the driver of the bus, Khushal Khan, told reporters at the scene of the blast.

"They were eight to 10 men and they were carrying rocket launchers and Kalashnikovs."

Some shoes of the victims were scattered in the area. Rescue workers removed bodies from the spot.

Sunni militants have stepped up attacks against Shi'ites 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 Shi'ites were killed in a suicide bombing near Quetta on the Muslim festival of Eid on August 31.

Pakistan has seen a surge in violence since al-Qaida 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.