Militants fighting for the ISIS in Syria have released sickening footage of the murder of a man the terror group accused of working as a spy, Daily Mail reports.

The video was filmed in Aleppo and shows a man believed to be named Ahmed Muhammad being led out to a forecourt by masked executioners, who then shoot him in the head multiple times at point blank range.

In a chilling sign of ISIS' reach and influence, Muhammad is understood to have been accused of working as part of a secret spy network based in neighboring NATO member state Turkey before his brutal filmed execution over the border in Syria.

Titled 'A Lesson To Be Learned', the three minute 40 second video has all the hallmarks of an official ISIS release - employing HD cameras, Hollywood-style sound and visual effects and slick editing and direction.

It also carries the logo of one of the group's official Aleppo-based propaganda arms, Halab.

But in Iraq the ISIS terrorist group released a video on Monday in which four Sunni tribesmen from the Tikrit region are executed for allegedly collaborating with the government.

The video was released on the day some 30,000 government forces launched a huge operation to retake Tikrit, which the terrorists have controlled for almost nine months.

The video describes them as belonging to a tribal group named after a town just north of Tikrit.

"Elimination of a Sahwat al-Alam cell," reads a banner just before masked gunmen are seen shooting a bullet into the back of each victim's head.

ISIS released pictures of the execution of 13 alleged members of the group in December.

According to military sources, several units of Sunni tribal fighters were involved in the offensive launched on Monday, alongside the army, police, Shiite volunteers and militias.

Announcing the operation on Sunday, Prime Minister Haider al-Abadi urged all Tikrit resident to lay down their arms and turn against ISIS.