"I swear to God we will eat your hearts out, you soldiers of Bashar. You dogs. God is greater!" the rebel fighter was quoted as saying.
"Heroes of Baba Amr ... we will take out their hearts to eat them."
A group loyal to Syrian President Bashar al-Assad posted the video online. The group described the mutilation as a "crime that crosses all lines".
The sentiment was shared by the main opposition alliance, which described the act as "horrific and inhumane". Rebels who are seeking to oust the government of President Assad are backed financially and militarily by the West.
"The Syrian Coalition strongly condemns this act, if it is revealed to be true," the dissident group said in a statement.
"The coalition stresses that such an act contradicts the morals of the Syrian people, as well as the values and principles of the (rebel) Free Syrian Army," it said.
Though agencies could not verify the authenticity of the video, it interviewed a rebel spokesman who confirmed the incident and said he spoke to the man in the footage.
Tariq al Sayed, spokesman from the Homs city neighbourhood of Baba Amr, said he is a friend of the rebel shown in the video.He said the incident took place more than two weeks ago, after several rebels and government troops were killed in a battle in Homs.
Al Sayed said when he saw the video, he told his friend to take it off the internet because the act was so perverse.
"This was an isolated incident. (His) actions do not represent the FSA. His actions only represent himself," al Sayed told agencies.
"This is not just a normal person who sits home. He has had two brothers killed. His mom and dad were detained, and the rest of his family displaced," he said.
Human Rights Watch said the atrocious act in the video was inexcusable.
"It is not enough for Syria's opposition to condemn such behaviour or blame it on violence by the government," said Nadim Houry, Middle East deputy director at HRW.
"The opposition forces need to act firmly to stop such abuses."