A bombing by the Islamic State group in the city of Ramadi last week killed 13 Iraqi soldiers, defence ministry said Thursday, denying reports they died in an air strike.

The March 11 blast completely destroyed a house used by the army in Anbar provincial capital Ramadi, west of Baghdad, and damaged neighbouring homes as well, the ministry said, announcing the results of a probe.

"The bodies of 13 of our heroic martyrs were found lying at various distances from the site of the explosion," it said, without specifying whether this was the final toll or if more might still be missing.

The blast occurred from the bottom up and there was evidence of a tunnel leading to the house, used as the headquarters of an army company, from the direction of IS lines, it said.

The ministry rejected reports that the blast was caused by an air strike, saying that neither Iraqi aircraft nor those from a US-led coalition carrying out strikes against IS were active in the area at the time.

IS launched a series of 23 bombings in Ramadi that day, it said.

Anti-government fighters have held parts of Ramadi for more than 14 months.

The city has seen heavy fighting between security forces and IS, which spearheaded a sweeping offensive last June that overran large areas north and west of Baghdad.