Iraqi security forces have found mass graves in an area recently retaken from ISIS that could contain up to 400 bodies, an Iraqi official says.

The bodies of civilians and security forces were found in an abandoned base near Hawija, a northern town retaken in early October, said Kirkuk​'s governor, Rakan Saed. He didn't say when authorities will start exhuming the bodies from the mass graves.

Khalaf Luhaibi, a local shepherd who led troops to the site, said the Islamic State in Iraq and Syria 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.

Iraqi forces have driven ISIS from nearly all the territory it once controlled. Authorities have already uncovered several mass graves in other newly liberated areas.

U.S.-backed Iraqi forces have driven the extremists from nearly all the territory they once controlled, with some fighting still underway near the western border with Syria.

On Saturday, Iraq's prime minister announced an operation to capture a patch of territory on the western edge of the country near the border with Syria. Hours later, Iraqi Defence Ministry announced capturing Romana area, saying the troops will head to the nearby town of Rawa.

According to Ahmed al-Asadi, a spokesman for the Shia-majority paramilitary forces, Rawa is the last Iraqi town held by ISIS but the group still controls some scattered small villages in mainly desert areas.