(CNN) -- The ghastly video shows how barbaric the Syrian civil war can be.
A man, said to be a well-known rebel fighter, carves into the body of a government soldier and cuts out his heart and liver.
"I swear to God we will eat your hearts out, you soldiers of Bashar. You dogs. God is greater!" the man says. "Heroes of Baba Amr ... we will take out their hearts to eat them."
He then puts the heart in his mouth and takes a bite.
A group loyal to President Bashar al-Assad posted the video online Monday. The group describes the mutilation as a "crime that crosses all lines."
It's a sentiment shared by the main opposition alliance, which describes the act as "horrific and inhumane."
"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."
On Wednesday, the FSA said it will investigate whether the man in the video is a member of the rebel army.
In a Facebook post, the group also said "the abuser will be punished severely even if they are associated with the Free Syrian Army."
Rebel spokesman: There's more to the story
Although CNN cannot independently verify the authenticity of the video, CNN has interviewed a local rebel spokesman who confirmed the incident and said he has spoken to the man in the footage.
Tariq al Sayed, a spokesman from the Homs neighborhood of Baba Amr, said he is a friend of the rebel 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 western 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 said. "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."
The Baba Amr district of Homs, once a bastion of anti-government sentiment, was subjected to a brutal counteroffensive by the Syrian army starting in February 2012, Human Rights Watch said.
Homs came under weeks of relentless attacks by government forces, including indiscriminate shelling on civilian areas.
But the government has repeatedly denied attacking civilians, saying Syrian forces were targeting armed gangs and foreign terrorists bent on destabilizing the government.
Regardless of the horrors suffered in Homs, the atrocious act in the video is inexcusable, 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."
CNN's Hamdi Alkhshali and Saad Abedine contributed to this report.