A large mass grave believed to contain bodies of thousands of Iraqi security forces killed by ISIS has been discovered not far from the strategic city of Mosul, where government forces and their allies are fighting the terrorist group in an operation to retake Mosul.

The mass grave, which is considered to be the biggest one in conflict-ridden Iraq, is located some 400 kilometers north of the capital Baghdad.

The Khasfa sinkhole is the resting place of an estimated 4,000 victims, according to a report by the British daily newspaper The Daily Telegraph.

The report says that most of the ill-fated Iraqi policemen were shot and dumped into the natural depression just off the Baghdad-Mosul highway, while others lost their lives in vehicles driven over the edge of the pit.

Mahmoud, a 40-year-old resident of the nearby Sananik village, says that ISIS “would drive the victims to Khasfa in convoys of minibuses, trucks and pickups. The men had their hands bound and their eyes blindfolded.”

“They were taken to the sinkhole and shot in the back of the head,” he added.

“Khasfa is definitely one of the biggest, if not the biggest, mass grave by Daesh in Iraq. And from what we have heard it is not just a grave but also an execution site,” Belkis Wille, senior Iraq researcher for Human Rights Watch, said.