The incident occurred last night in Machh area of Bolan district when rebels stopped vehicles going from Quetta to Punjab province and took 26 passengers hostages.
"They took away 13 of them to nearby mountains and killed them," Niaz Ahmad from local police station in Machh told PTI on telephone.
"The rest of passengers were set free and allowed to go."
He said in a separate incident in the same area rebels killed one soldier of para military Frontier Corps (FC).
Balochistan Liberation Army, a separatist militant group, took responsibility of the attack.
The Baloch nationalists accuse federal government of usurping the natural resources of the province like natural gas and using them for the benefit of Punjab province.