An explosion targeting a police bus in Pakistan's southwestern Balochistan province killed at least three officers and wounded 16 others yesterday.

The bus was carrying around 40 police officers when it was hit by the blast in Mastung district, around 40 kilometres south of the provincial capital Quetta, local administration official Raja Muhammad Akram told AFP.

"It was a roadside IED (improvised explosive device) blast that resulted in the deaths of three policemen while wounding 16 others," Akram said.

Pakistan has been battling a separatist insurgency in Balochistan for decades, where militants target state forces, foreign nationals, and non-locals in the mineral-rich southwestern province bordering Afghanistan and Iran.