(CNN) -- Fifteen passengers were burned alive when a passenger bus was set on fire by unknown men in Pakistan's volatile southwestern Balochistan province, an official said.

Naseer Ahmed Nasir, deputy commissioner in the city of Sibi, said four men on motorcycles initially opened fire on a parked passenger bus at a hotel. "The armed men then came close to the bus and sprinkled petrol, setting it on fire," he said.

He said most of the dead were either children or women.

Balochistan, near the Afghan border, is rich in natural resources. But it is also battling an insurgency.

Militants there have for years complained that the central government has paid little attention to them and their economic needs. They have been fighting Pakistani military forces for self-rule.

Pakistan has said the militants in Balochistan do not have links to the Islamic hard-liners who have launched assaults in other parts of the country.