Monday's bombing in Baluchistan province's Mastung district also injured 25 people, many critically, KarachiNews.net reported.
Witnesses said the car was driven by a suicide bomber who detonated the explosives as the buses were passing another vehicle near Quetta, capital of Baluchistan province, close to the Iranian border.
Officials said women and children were among the victims. A government official in Quetta said the buses were taking about 180 Shiite Muslim pilgrims to Iran.
Also Monday, seven health workers, six of them women, were shot and killed near the Swabi district, Dawn reported.
Police said a vehicle carrying the Support With Working Solutions workers was traveling home from a school and health center when it was ambushed near Anbar Interchange in Swabi district.
Authorities said gunmen, likely riding motorbikes, sprayed the vehicle with bullets.
A motive has not been determined, police said, adding that the matter remained under investigation.
Pakistani Taliban previously threatened health programs, such as polio eradication, claiming they are a cover for U.S. spying. Suspicion is based on the case of Shakil Afridi, a tribal area doctor who was paid by the CIA to run a sham hepatitis vaccination campaign near Osama bin Laden's compound in Abbottabad before the May 2011 U.S. commando raid that killed the al-Qaida founder.