A mortar bomb blast inside a school in northern Afghanistan Thursday killed at least nine students, police said.

"As per initial investigation, the explosion was caused by a mortar that had somehow been carried inside the madrassa," police spokesman Khalil Asir told Agence France-Presse (AFP), adding many of the dead were aged under 18.

The blast happened in Ishkamish district of Takhar province.

Provincial governor spokesman Jawad Hejri confirmed the incident.