"An arms depot exploded... The security forces have opened an inquiry to determine the cause," Baghdad's security operations command said in a statement.

Seven people were killed and 22 injured in the blast, a senior police source told AFP on condition of anonymity.

It was heavy weapons, including rocket-propelled grenades and shells, belonging to an armed group and stored in a house, he said.

The sources said the blast struck near a Shiite mosque in the district of northeast Baghdad.

Witnesses reported heavy material damage to homes and other buildings in the Sadr City district that is a stronghold of populist Shiite cleric Moqtada Sadr.