At least nine people, including five schoolchildren, have been killed and 23 injured in a bombing near a girls’ school in southwestern Pakistan, officials said.

Friday’s attack targeted police guarding a polio vaccination drive in Mastung, a town in Balochistan province.

“The target was a police van which was going to pick up a polio [vaccination] team,” Senior Superintendent of Police Rahmat Ullah told the Reuters news agency.

The five children, as well as one police officer and a passerby, were killed immediately in the explosion, sources told Al Jazeera. Two men died later in hospital due to critical injuries.

The blast was believed to have been caused by an improvised device attached to a motorcycle parked near the school.