Tehran: Iranian authorities on Monday continued investigating the cause of a explosion that killed at least 12 people and wounded more than 200 others at a mosque.

Several officials have insisted the blast in Shiraz late on Saturday was an accident and not a bomb attack, but others said it was sabotage or negligence over the handling of decades-old ordnance inside the mosque.

The state inspection organisation and the justice ministry are heading the investigation, according to the state broadcaster.

"We do not want the results of the investigation to be announced in a hurry and without the necessary precision," Shiraz governor Ebrahim Azizi said. "We want the work of the experts to be based on facts and realities."

The blast went off just after a prayer sermon by prominent local cleric Mohammad Anjavinejad.