The 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 terrorists forums, which included both new and previously released footage, shows hundreds of executions, providing further evidence of the scope of the atrocity.

The grisly footage shows executions on an industrial scale, with victims falling out of dump trucks and later lying side by side in shallow mass graves before being shot dead one by one.

The killing went on into the night and the video shows an excavator being used to move piles of bodies.

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.

"This is a message I address to the whole world and especially to the Rafidha ..., 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.

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 terrorists, AFP reports.