The sickening mass execution video was posted online by an anti-ISIS activist based in Yemen and quickly started to draw condemnation from social media users shocked by the atrocities.

However the footage is believed to have filmed in late August 2014 and the victims are prisoners of war who were caught by ISIS after the jihadi group overran the Assad held Tabqa airbase in Syria.

Forced to lie side-by-side and with their faces in the dirt, the video shows about 200 Assad regime soldiers fighting in the Syrian Arab Army stretched out in a long line to the left and right.

Behind them, at least a dozen gunmen armed with assault rifles and handguns stand poised and ready to fire. 

One militant then begins firing at point blank range - shooting each prisoner individually as he makes along the row of victims.

Others beside him also open fire, mostly using their machine guns, and the area is filled with dust, obscuring much of the bloodshed. 

The video is believed to have been filmed close to Tabqa airbase after the strategic site was overrun by militants.

ISIS continues to control a swathe of territory, including Raqqa, the capital of the terrorists' self-proclaimed caliphate.   

ISIS is known to have carried out mass executions of prisoners in the past. In one instance, 1,700 Iraqi military cadets were slaughtered in a mass killing that lasted an entire day and night.

Grisly footage of the killings - likely to rank among the group's worst atrocities - showed executions on an industrial scale near Tikrit in Iraq.

Young men were filmed falling from trucks and pleading for their lives before being lay down in shallow graves and sprayed with bullets.

(dailymail.co.uk)