PUL-I-ALAM, Afghanistan (Reuters) - An explosion outside a school south of the Afghan capital on Thursday killed at least 15 people, including 12 students, an official said.

The blast happened as a police convoy patrolled in Mohammad Agha district of Logar province and was caused by explosive materials in a truck, the district chief said.

"So far I can tell that 12 students, three police and several civilians have been killed," Abdul Hamid told Reuters by phone.

The age of the students was not immediately known.

The truck, which was carrying explosive materials, had rolled over into a stream overnight, Hamid said.

It was not clear whether the blast was a sabotage act amid increasing violence in Afghanistan or was an accident. Afghans use explosive materials for construction, mining works and even for hunting.

Some people were also wounded by the blast, Hamid said.

(Writing Sayed Salahuddin; Editing by Sanjeev Miglani)