The video is one of several posted in recent months that appear to show people being killed or dead bodies mutilated, but the perpetrators usually are members of pro-government paramilitary groups called the shabiha, The New York Times reported Monday.
The video posted Sunday was the first that showed apparent cannibalism.
In it, a man identified as Abu Sakkar, reported to be a rebel commander, is shown removing the heart and liver of what appears to be a dead government soldier and threatening members of the country's Alawite religious minority, the sect of President Bashar Assad.
"I swear to God, soldiers of Bashar, you dogs -- we will eat your heart and livers!" Abu Sakkar said in the video. "God is great! Oh, my heroes of Baba Amr [a Homs neighborhood], you slaughter the Alawites and take their hearts out to eat them!"
The man then looks at the organs and holds them closer to his mouth, as if to bite them, the Times said.
In a statement Monday, Human Rights Watch described the mutilation portrayed in the video as an atrocity and said the man who apparently committed it was a leader of the Independent Omar al-Farouq Brigade.
Human Rights Watch has accused the Omar al-Farouq Brigade of participating in the "indiscriminate shelling" of two Shiite villages in Lebanon in April, but said it was unclear if the group operated as part of the Free Syrian Army.
By comparing frames of the mutilation video to other videos showing what appears to be the same man, Human Rights Watch said in a release it believes the person in the video to be Abu Sakkar. Journalists and other commanders have said that Abu Sakkar is the nom de guerre of a former commander from the mainstream al-Farouq Brigade from the Baba Amr district of Homs.
Asked about the video, Maj. Gen. Salim Idriss, chief of staff for the Supreme Military Council of the opposition, told Time magazine "such violence is unacceptable, and no soldier under the council's command would be allowed to get away with such actions."
Opposition forces have not established accountability protocols for abuses committed by their members, Human Rights Watch said.
"It is not enough for Syria's opposition to condemn such behavior or blame it on violence by the government," said Nadim Houry, Middle East deputy director at Human Rights Watch. "The opposition forces need to act firmly to stop such abuses."