In the video, Abu Sakkar, a founder of the rebel Farouq Brigade, begins cutting apart the body of a dead Syrian soldier.
Then, according to Reuters, he addresses the camera, saying: "I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog."
He eats the heart of the dead soldier, while the crowd behind the camera cheers "Allahu akbar" ("God is great").
The video is branded with the logo of the Syrian Truth Facebook page.
The Syrian Truth Facebook page posted the video to YouTube, where it is still available. On Facebook, the video has almost 2,000 shares.
Human Rights Watch has condemned the video, saying it is not only a war crime, but also a frightening example of Syria's descent into sectarian violence.
"The mutilation of the bodies of enemies is a war crime. But the even more serious issue is the very rapid descent into sectarian rhetoric and violence," said Bouckaert.
He said that in the unedited version of the film, Abu Sakkar instructs his men to "slaughter the Alawites and take their hearts out to eat them", before biting into the heart.