The grisly footage shows executions on an industrial scale, as young men are seen falling from trucks and pleading for their lives before being lay down in shallow graves and sprayed with bullets.

Others are shot individually and their bodies dumped into the Tigris River, while an excavator is seen shifting vast piles of bodies as the executions continue into the night.

The footage reveals the true scale of last year's Speicher massacre, in Tikrit, Iraq, where up to 1,700 military cadets are thought to have been slaughtered.

If body count estimates are correct, the killing of the mostly Shiite Muslims in June last year ranks as one of the worst atrocities seen in Iraq in the past decade.

The wholesale slaughter was so vast the graves used to bury the victims could be seen from space.

Highest estimates put at 1,700 the number of cadets ISIS gunmen captured at the Speicher military base near Tikrit and executed at various locations, mostly in the city's former presidential palace complex.

The 22-minute video, posted on jihadist forums, shows some victims pleading for their lives, attempting to explain they had only just joined the security forces.

Around 600 bodies have been exhumed since government and allied fighters retook Tikrit from ISIS in April but many of the victims were dumped into the Tigris river.

An unidentified ISIS leader in military uniform is seen in the video released on Saturday.

He states: 'This is a message I address to the whole world and especially to the Rafidha dogs, I tell them we are coming,' he said, using the pejorative term ISIS employs for Shiite Muslims.

The video was released four days after a court in Baghdad sentenced 24 men to death by hanging over the Speicher massacre.

The trial lasted only a few hours, and the convictions were based mostly on confessions the defendants claimed were obtained under torture.

Combined with a call by the country's top Shiite cleric Grand Ayatollah Ali al-Sistani for Iraqis to take up arms against them, the Speicher massacre played a key role in the mass recruitment of Shiite volunteers to fight the jihadists.

One of the spots where the Speicher cadets were executed is a police building in the sprawling Tikrit palace complex former president Saddam Hussein built in his hometown.

The quay where the victims were shot in the head and pushed into the Tigris has, since Tikrit was retaken, been turned into an improvised shrine.

Relatives, many of whom may never have a body to bury, have streamed to the site over the past two months along with fighters, delegations of officials, students and others. 

(dailymail.co.uk)