A powerful bomb attached to a motorcycle exploded near a vehicle carrying police assigned to protect polio workers in south-west Pakistan on Friday, killing nine people including five nearby children, and wounding 17 others, officials said.

Local police chief Fateh Mohammad said the attack happened in Mastung, a district in Balochistan province.

He said a motorised rickshaw carrying schoolchildren was nearby when the bomb went off, resulting in the deaths of five children, a police officer and two passersby.

A boy on a hospital bed
A boy who was injured in the explosion in Mastung is treated at a hospital in Quetta (Arshad Butt/AP)

Some of the wounded were moved to a hospital in Quetta, the capital of Balochistan province, said Wasim Baig, a spokesman for the health department. He said seven people died on the spot and two men who had been critically wounded died in hospital.

No-one claimed responsibility for the attack on police, but suspicion is likely to fall on separatist groups and Pakistani Taliban which have stepped up attacks on security forces and civilians in recent months.

The police chief in Mastung, Rehmat Ullah, said a police van came under attack when it was heading to a health centre to escort polio workers for the door-to-door campaign that began on Monday to vaccinate 45 million children under five, following a surge in new cases.

Pakistan has recorded 41 polio cases across 71 districts so far this year.

The latest attack came days after militants attacked a health centre used in the ongoing anti-polio campaign in the north-western district of Orakzai, triggering a shootout that left two policemen dead. Three of the attackers were also killed.