A mass grave believed to contain the bodies of thousands of Iraqi security personnel killed at the hands of the Daesh Takfiri militants has been discovered not far from the strategic northern city of Mosul, where government forces and their allies are engaged in the last phase of an operation to drive the extremists out of their last major stronghold in the country.

The Khasfa sinkhole lies eight kilometers outside Mosul, located some 400 kilometers north of the capital Baghdad, and is considered to be the biggest mass grave in conflict-ridden Iraq as it is the resting place of an estimated 4,000 victims, the British daily newspaper The Daily Telegraph reported on Sunday.

The report, citing witnesses, wrote 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.

“Daesh would drive the victims to Khasfa in convoys of minibuses, trucks and pickups. The men had their hands bound and their eyes blindfolded,” said Mahmoud, a 40-year-old resident of the nearby Sananik village, who added, “They were taken to the sinkhole and shot in the back of the head.”

The dead either tumbled into the hole after being shot or were tossed into it by their masked killers, Mahmoud pointed out.

The body count at Khasfa mass grave outnumbers any other known sites in Iraq, especially the murder of about 1,700 Iraqi Air Force cadets in a June 2014 attack on Camp Speicher military base near the northern city of Tikrit.

A soldier prays at a mass grave for Iraqi soldiers from Camp Speicher who were killed by Daesh militants in the presidential compound of the former Iraqi dictator, Saddam Hussein, in Tikrit, Iraq, April 6, 2015. (Photo by Reuters)

“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, a senior Iraq researcher for Human Rights Watch, said.

The sinkhole is said to be roughly 400 meters deep but contained so many bodies they began to be visible from its periphery.

“In the beginning, you couldn't see the corpses at the bottom of the hole. Only later, when it began to fill up, you could see the bodies,” Mahmoud said.

Some 2,000 policemen and soldiers were murdered by Daesh in one day alone, he claimed, adding that the victims of one of the executions he was forced to watch included his cousin, who worked as a police officer.

Daesh militants overran Mosul in June 2014. Iraqi government forces, backed by fighters from allied Popular Mobilization Units, better known by the Arabic word Hasd al-Sha’abi, managed to liberate the eastern quarter of Mosul from the clutches of Daesh terrorists in January, three months after launching the operation to regain control of the country’s second-largest city.

On February 19, Iraqi soldiers and Hashd al-Sha’abi fighters mounted a new offensive to liberate western Mosul.

International aid organizations have warned against the mass exodus of hundreds of thousands of civilians from western Mosul.

Makeshift camps are being built up in nine cities near Mosul and tents are being pitched in existing camps to accommodate up to 400,000 internally displaced people.