Footage has emerged showing a rebel Syrian fighter carving out what appears to be the heart of a regime soldier and putting it in his mouth, Human Rights Watch says.
The video, which has been posted on the internet, shows the fighter standing over the body of a uniformed soldier holding what the organisation says appears to be a heart and liver while saying: "We will eat your heart and livers".
Human Rights Watch said the man appears to be a commander of the Syrian opposition Independent Omar al Farouq Brigade whose nom du guerre is Abu Sakkar.
The brigade is also said to have been behind the indiscriminate shelling of Lebanese Shia villages in April.
The Syrian opposition National Coalition has condemned the act, which if proven would be classed as a war crime by the International Criminal Court.
Human Rights Watch has urged all parties to keep weapons from reaching the Independent Omar al Farouq Brigade.
Nadim Houry, the Middle East deputy director at Human Rights Watch said: "One important way to stop Syria's daily horrors, from beheadings to mutilations to executions, is to strip all sides from their sense of impunity.
"These atrocities are shocking but so is the obstruction of some Security Council members that still do not support an ICC referral for all sides."
A statement from Syria's National Coalition said: "The Syrian Coalition strongly condemns this act - if it is revealed to be true.
"The Coalition stresses that such an act contradicts the morals of the Syrian people, as well as the values and principles of the Free Syrian Army."
According to Human Rights Watch, Commander Abu Sakkar is a former commander from the mainstream al Farouq Brigade from the Baba Amr district of Homs, an early stronghold for rebel fighters.
Homs was subjected to a brutal counter-offensive by President Bashar al-Assad's forces in February 2012.
According to the Independent Omar al Farouq Brigade website, the group was formed in October 2012 with fighters that fought in Baba Amr.
The link between the brigade and the main fighters of the Syrian opposition is unclear.
A statement released by the British Foreign and Commonwealth Office said: "We are aware of a video circulating online that purports to show a member of the Syrian armed opposition committing an act of barbarism on a regime soldier.
"We join the Syrian opposition National Coalition in condemning this act - there should be is no place for these kinds of acts in Syria.
"The National Coalition has committed to reaching out to minorities and has made clear that there will be a place for all Syrians, regardless of ethnicity or religious belief, in the future Syria.
"It is time that extremist groups in Syria understood that the Syrian population does not want them there.
"This is why the UK is intensifying our support to the moderate opposition to build their credibility as an alternative to the Assad regime and to counter extremist influences in the Syrian conflict."
In the footage Abu Sakkar is seen holding the corpse’s heart and liver aloft, while also brandishing a dagger.
The person filming the act identifies the commander and says: "God Bless you, Abu Sakkar, you look like you are drawing (carving) a heart of love on him."
Abu Sakkar then speaks into the camera and says: "I swear to God, soldiers of Bashar, you dogs – we will eat your heart and livers! Takbir! God is great. Oh my heroes of Baba Amr, you slaughter the Alawites and take their hearts out to eat them."
However, TIME claims to have spoken to Abu Sakkar, whose real name is Khalid al Hamad in an interview by Skype. He claims he has no regrets about sinking his teeth into an organ that he said at the time believe was a liver.
He said that he carried out the act because he had found footage on the soldier's phone showing that soldier using a stick to abuse and humiliate a woman and her two daughters who had all been stripped naked.
The two-year civil war has claimed more than 80,000 lives and displaced five million people.
Both sides have been accused of using illicit chemical weapons, including the nerve agent sarin, against each other.
On Monday, Prime Minister David Cameron and US President Barack Obama pledged action to end the bloodshed in Syria, including providing more money for non-lethal equipment for rebel forces and humanitarian aid.
They also pushed the international community to help with the creation of a recognisable and viable Syrian opposition that would be capable of taking power in the event Assad's regime falls.