Authorities said separatist rebels opened fire on the buses after the car bombing.
The outlawed Baloch Liberation Army claimed responsibility for the attack.
A military statement said “sanitization operations in the area would continue and perpetrators of this henious and cowardly act will be brought to justice.”
Last week, rebels attacked a train and took about 400 people hostage. Twenty-six died before security forces launched an operation that killed all the 33 attackers.
Sarfraz Bugti, the chief minister of Balochistan, said security forces returned fire on Sunday and killed at least four of the attackers.
Oil- and mineral-rich Balochistan is Pakistan’s largest and least populated province. Ethnic Baloch residents have long accused the central government of discrimination, a charge Islamabad denies. The Baloch Liberation Army has been demanding independence from the central government.