A colonel in the police of Anbar, where Fallujah is located, said the mass burial site was discovered in the town of Saqlawiya, northwest of Fallujah in the Euphrates Valley.
"The security forces of the federal police, the army and the Hashed al-Shaabi found a mass grave in the Shuhada neighborhood during a mine clearing operation," he said AFP reported on Monday.
"The mass grave contains about 400 bodies of members of the military. There are also some civilians," he said, speaking on condition of anonymity because he is not authorized to talk to the press.
The colonel said most of the victims appeared to have been shot in the head.
"The security forces have opened the mass grave and started transferring the bodies for identification," he said.
The bodies are believed to belong mostly to Iraqi soldiers who were massacred by Daesh (ISIL) in a series of devastating attacks on army bases in the area.
"The organization (Daesh) executed many members of the military, as well as civilians, in this area in late 2014 and early 2015," he said.
Rajeh Barakat, a member of the Anbar provincial council, confirmed the discovery.
"The mass grave also includes civilians executed by Daesh on various charges such as spying or breaking the organization’s rules,” he said.
Iraqi forces launched an operation to retake Fallujah – one of the terrorists' most emblematic bastions – two weeks ago.
The offensive included operations to seal the siege of Fallujah by retaking the smaller town of Saqlawiya and its surroundings, a phase which has nearly been completed.