Mass graves discovered at an Iraqi airbase that was previously under the Islamic State group’s control could contain up to 400 bodies, an Iraqi official said Sunday after local residents described the atrocities the terror group carried out.

The bodies were found in an abandoned base near Hawija, a northern town retaken in early October, Kirkuk governor Rakan Saed said, adding that the place was “turned into an execution ground.” Hawija, about 150 miles north of Baghdad, was under ISIS control since 2013 until last month.

Officials discovered torn clothing and possible human bones and skulls in the area. It’s unclear when the bodies, which include civilians and security forces, will be exhumed.

Khalaf Luhaibi, a local shepherd, led troops to the mass graves and said ISIS brought captives to the area, shot and killed them or poured oil over them and lit them on fire. Other residents also gave “witness accounts” of the atrocities ISIS carried out when it held control of the area, the BBC reported.

ISIS’ territory in Iraq has diminished significantly after Iraqi forces drove its fighters out of two key cities — Mosul and Raqqa — earlier this year. Iraqi forces have driven ISIS from nearly all of its territory, uncovering several mass graves in other newly liberated areas.

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 Defense Ministry announced capturing Romana area, saying the troops will head to nearby town of Rawa.

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

The Associated Press contributed to this report.