Geo News reported the bomb, believed planted on a rickshaw, tore through the vehicle carrying Pakistani paramilitary personnel. More than a dozen people, most of them security personnel, were injured in the attack.
Dawn newspaper said those killed included nine security men. The attack occurred in the provincial capital's busy Bhosa Mandi area.
The report quoted police as saying the explosive device was set off remotely. The injured were taken to nearby hospitals, where some were reported in critical condition.
No group had yet claimed responsibility.
Balochistan province has been the scene of much sectarian, militant and separatist violence for years. It is home to a large population of Shiite Muslims, a minority in Sunni Muslim Pakistan.
On May 12, a suicide car bomber in Quetta killed six people and injured about 50 more, several of them seriously. The attack had targeted the police inspector general of the province, but he escaped unhurt.
Last January, in one of the worst acts of sectarian violence also in Quetta, about 100 people, mostly Hazara Shiites, were killed in two separate explosions. The banned Sunni militant group Lashkar-e-Jhangvi, associated with militant groups and blamed for similar attacks in the past, later claimed responsibility.