Mohsin Ali Butt, head of the Belgian provincial police, told CNN that 15 to 20 attackers had stopped the buses near the town of Ormara in the south of the province.
The militants ordered 14 of the total passengers to disembark and shoot at close range before fleeing. "This was an act of deliberate killing," he said.
An alliance of Baloch separatists, Baloch Raaji Aajoi Sangar (BRAS), declared responsibility for the killings. He said in a statement to journalists that they had attacked passengers who were part of the country's military.
In the statement, a BRAS spokesman said, "The fatalities carried ID cards from the Pakistani Navy and the Coast Guard and were killed after they were identified."
The buses were traveling Pakistan's largest city Karachi to the port city of Gwadar. Police and other government agencies have launched investigations into the killings, Butt said.
Prime Minister Imran Khan expressed his condolences to the families of the victims and promised to "bring the perpetrators of the barbaric act to justice".
Last Friday's attack hit the city of Hazarganji district, home to a large community of the Hazia Shia Muslim minority. Lashkar-e-Jhangvi, a Sunni militant group known to attack Hazaras, said she was responsible.
In December 201
In July last year, 149 people were killed in a suicide attack in the Mastung area south of Quetta and 186 injured convoy of a political candidate.
ISIS took responsibility for this bomb attack, the second most deadly terrorist attack in Pakistan's history.
Journalists Syed Ali Shah of Quetta and Saleem Mehsud of Islamabad contributed to the report.