Gunmen disguised as members of the Pakistani security forces killed at least 22 passengers on Friday night after forcing them off buses traveling from the western city of Quetta to Karachi on the southern coast, Reuters reported.
The assault in the province of Baluchistan occurred in the town of Mastung, around 40 km (25 miles) south of Quetta.
"Fifteen to 20 armed men in three pickup trucks and wearing security uniforms kidnapped around 35 passengers," Sarfaraz Bugti, Baluchistan's home minister, told Reuters.
He said the bodies of 22 passengers were later found around two km away from the main Quetta-Karachi highway in foothills. Seven of the assailants have been killed in an operation involving hundreds of soldiers and police, he added.
The circumstances of the passengers' deaths could not immediately be established and the motives of the assailants were unclear.
The attack will be a concern for the government of Prime Minister Nawaz Sharif because it raises further questions about the feasibility of a new economic corridor Pakistan wants to build with billions of dollars of Chinese investment.
The much-vaunted project, announced when Chinese President Xi Jinping visited Pakistan in April, envisages an eastern and western route, with the latter passing from Gwadar port in the south through Quetta and beyond.