Islamabad - Gunmen killed at least 19 passengers they had forced off buses travelling from the western Pakistani city of Quetta to Karachi on the southern coast, said the home minister for the restive province of Baluchistan, where the attack took place.
The assault occurred late on Friday in the town of Mastung, around 40 km south of Quetta.
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"The armed men were wearing the uniforms of the security forces," Sarfaraz Bugti told Reuters. The bodies of 19 passengers had been found so far, he said.
Security officials said around 25 passengers were taken off two buses, and an operation began to find them. The bodies were discovered in nearby hills, the officials said.
The circumstances of their deaths could not immediately be established, and the motives of the assailants was unclear.
Separatists have been fighting a low-intensity insurgency in Baluchistan, of which Quetta is the capital, for decades. They are demanding an end to what they see as the exploitation of their resources by people from other parts of Pakistan.
Islamist militants also regularly attack civilian and military targets, and earlier this month at least 43 commuters were killed on a bus in Karachi by a group that has declared allegiance to Islamic State.
All of the victims in that attack were Ismailis from Pakistan's minority Shi'ite community, but one security official said the Mastung incident was unlikely to have been a sectarian attack.
As well as the 19 killed, one person was injured and five more people rescued unharmed.