Parts of a sickening video released by Islamic State militants that shows members of the terror group beheading 21 Coptic Christians have been faked, experts have claimed.

The footage, which lasts five minutes, shows the Egyptian Christians dressed in jumpsuits being marched one by one along a lonely beach, each held by a fighter clad in black.

The captives, their faces uncovered, are made to kneel before being forced to lie down. The masked jihadists then behead them simultaneously.

While experts believe the men were killed by the terrorists, questions have been raised over whether some scenes - including one where the militants appear to be 7ft tall - have been manipulated.

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Doctored? The executioners appear to be seven-foot tall in this still taken from the sickening footage

Experts believe that the scene of ISIS terrorists marching 21 Coptic Christians to their death was faked

It is believed the actual murders were filmed in a different location and the sea was added at a later stage

There are also doubts as to whether the men were all murdered on the beach and whether they were all killed at the same time.

It is thought sections of the footage might have been shot on a 'green screen' in a studio - a technique used in Hollywood blockbusters - and that the beach background was added at a later stage.

Veryan Khan, of the Florida-based Terrorism Research and Analysis Consortium, told Fox News that there are several technical mistakes in the video that show it was manipulated.

She said that in the shot of the terrorists marching their prisoners along the beach, the jihadis appear to be 7ft tall - towering as much as two feet above their victims.

This observation was supported by Hollywood director Mary Lambert who described it as the shot with the 'really tall Jihadists and the dwarf Christians.'

Ms Khan added that the terrorist who speaks in the clip - dubbed 'Jihad Joseph' - appears much bigger than the sea in some shots, while his head looks out of proportion to his body.

The sickening image of blood in the sea is believed to have been created using special effects

Ms Khan explained that this meant it was likely he had been filmed indoors and the sea scene, believed to be in northern Libya, had been placed behind him at a later stage.

Meanwhile, Lambert said that the scene that apparently shows the sea turning red with the blood of the beheaded men 'was obviously special effects'.

The experts claimed that only a few Jihadis would have been on the beach along with a 'less talented crew' than the ones responsible for some of the group's more high-production videos.

Earlier this week, local media reported that militants from the Islamic State and Ansar Al-Sharia were understood to have rounded up dozens of farm workers in the wake of bombings by Cairo.

It raises the chilling prospect of yet another mass execution in what is being seen as ISIS's bid to announce its presence in a new region where it is gaining influence.

Initial reports said seven men had been seized, but that figure had risen to more than 35 by mid-afternoon yesterday, according to The Libya Herald.

It came as Egypt blitzed ISIS training camps, weapons stockpiles and fighters in two waves of air strikes following the gruesome murder of captured Egyptian workers.