Bodies of civilians and security forces were found at a base occupied by ISIL until early October
Mass graves discovered in Iraq's Hawija
Iraqi security forces have found mass graves in an area recently retaken from ISIL that could contain up to 400 bodies.
The bodies of civilians and security forces were found at an abandoned base near Hawija, a northern town in Kirkuk province that was occupied by ISIL fighters until early October, provincial governor Rakan Saed said.
Mr Saed did not say when authorities would start exhuming the bodies at the base, which he said had been "turned into an execution ground".
Khalaf Luhaibi, a local shepherd who led troops to the site, said: "ISIL used to bring captives to the area and shoot them dead or pour oil over them and light them on fire."
The area was strewn with torn clothing and what appeared to be human bones and skulls.
Authorities have uncovered several mass graves in other areas newly liberated from ISIL.
Iraqi forces are now fighting to retake one of the last areas of the country still held by the extremist group - a patch of territory on the western edge of the country near the border with Syria.
"Rawa is the last Iraqi town held by ISIL, who still control some scattered small villages in mainly desert areas," said a spokesman for the Shiite-majority paramilitary forces taking part in the operation alongside government troops.
Meanwhile, an Iraqi military helicopter crashed on Sunday during a training exercise, killing all three crew members on board, the defence ministry said.
The Russian-made Mi-17 crashed in Wasit province, the ministry said without disclosing the cause of the accident. It had initially reported that seven crew members died.
The province is about 160 kilometres south-east of Baghdad, far from the front lines of the war with ISIL.