A mass grave believed to contain 4,000 people has been discovered in the outskirts of Mosul, Iraq.

The Khasfa sinkhole was found five miles outside the northern city where US-backed Iraqi soldiers are in a months-long offensive against Isis.

Bodies are believed to have been dumped in the natural depression during 2014, according to local villagers, Iraqi police and human rights organisations.

Eyewitnesses have told a Telegraph reporter in Iraq how they saw victims executed at point-blank range.

"Daesh [Isis] would drive the victims to Khasfa in convoys of minibuses, trucks and pickups," a 40-year-old man named Mahmoud said to the newspaper. "The men had their hands bound and their eyes blindfolded. They were taken to the sinkhole and shot in the back of the head."

Western reporters and aid workers have been able to attain greater access to Mosul after much of the city have been reclaimed by Iraqi troops in recent weeks.

On 23 February, troops were regained control of Mosul Airport in what is seen as a strategic and symbolic victory.

The Khasfa mass grave is not the first time Isis massacres have been discovered as land held by the terror group has been reclaimed, but it is believed to be among the biggest.

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

The sinkhole is said to be roughly 400m 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," Mahmoud said. "Only later, when it began to fill up could you see the bodies."

The victims of one of the executions he was forced to watch included his cousin, who worked as a police office, he said.

mass grave
Members of the Iraqi security forces inspect an Isis mass grave in Tikrit (file picture). Getty