"The Iraqi army received reports that a cache of weapons and munitions had been discovered in an empty building in west Mosul," al-Jubouri said.
"The army arrived near the area and were waiting for the disposal experts to arrive when the explosion occurred."
Al-Jubouri said: "The force of the blast caused the building to collapse. Fifteen houses next to it were damaged."
US and Iraqi forces sealed off the area while rescuers hunted through the rubble for those caught up in the blast, he said.
Ninawa province is one of several northern regions where al-Qaeda in Iraq fighters have regrouped after being pushed out of Baghdad and western Anbar province last year.
Over the last month, the US military has been carrying out large-scale offensives in Iraq's northern provinces against al-Qaeda.