A remote-controlled bomb targeting a police van has killed three passers-by and wounded six other people in Pakistan's troubled southwest, police have said.

The explosive device detonated underneath a donkey cart on Sunday on the outskirts of Quetta, the capital of the oil-and-gas-rich Baluchistan province, which borders Iran and Afghanistan.

"The bomb targeting a police van exploded seconds after it passed by the cart, killing three passers-by and wounding six others including a policeman," Sikandar Tareen, a local police official, told the AFP news agency.

Another local police official, Saleem Shawani, confirmed the incident and casualties.

There was no claim of responsibility, but Baluchistan suffers from armed groups, sectarian violence between Sunni and Shia and a separatist insurgency.

Gunmen riding a motorcycle shot dead the chief warden of the district prison on May 19.

Al-Qaeda-linked militant group Lashkar-e-Jhangvi claimed responsibility and said "the slain warden was a cruel person and used to subject our imprisoned leaders and workers to torture".

Hundreds of civilians have been killed since Baluch rebels rose up in 2004 against the federal Pakistani government, demanding political autonomy and a greater share of profits from the resources in the region.