No group has claimed responsibility for the attack in Baluchistan, which has seen increased sectarian violence [Reuters]

Unidentified gunmen have opened fire on a bus travelling through southwest Pakistan, killing at least 25 Shia pilgrims aboard, officials have said.

The bus, which was on its way to the Iranian-Pakistani border, was attacked 30km away from the city of Quetta, located near the border with Afghanistan, Al Jazeera's Kamal Hyder reported from the Pakistani capital Islamabad.

Hyder said police authorities and district officials had reported that "the bus came under attack by unidentified attackers riding motorcycles, who riddled the vehicle with bullets".

The pilgrims were passing through the Mastung district in Baluchistan province, of which Quetta is the capital, when the attack occurred, local officials said.

About 40 pilgrims were riding on the bus and more than a dozen were wounded, Dadullah Baluch, a local tribal policeman, told the AP news agency.

A pickup truck blocked the vehicle's path, and a group of at least eight men carrying rockets and guns forced the passengers off, Khushhal Khan, the bus driver, said.

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

The passengers tried to run, but the gunmen opened fire, he said, after which they sped off in the truck.

The wounded laid on the ground for nearly an hour before rescue workers arrived, the driver said.

No group has claimed responsibilty for the attacks.

Sunni fighters have stepped up attacks against Shias in Baluchistan in recent months.

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

Officials have said there is no evidence linking them with Islamists.

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.

Most recently, at least 10 Shias were killed in a suicide bombing near Quetta on the Muslim festival of Eid on August 31.