The Free Syrian Army vowed on Wednesday to punish those committing atrocities, as anger mounted at a video showing a rebel filmed apparently cutting out and eating the organs of a soldier.
"Any act contrary to the values that the Syrian people have paid their blood and lost their homes to will not be tolerated, the abuser will be punished severely even if they are associated with the Free Syrian Army," the main rebel group said in a statement.
The gruesome video, in which the rebel fighter leans over a uniformed body, cuts out organs and then holds one up to his mouth, has prompted an outcry around the world and has thrown the spotlight on war crimes allegations against the opposition.
The Free Syrian Army (FSA) said field commanders had been instructed "to begin a prompt investigation into the matter in which the perpetrator will be brought to justice".
Investigations would also be held into whether the rebel in the video is a member of the Free Syrian Army or not, it said.
Time magazine reported that the Syrian rebel at the centre of the row has defended his actions as revenge for regime atrocities.
The magazine said it had talked by Skype with the fighter in the video, whom it identified as Khalid al-Hamad.
Hamad claimed he was driven to the gruesome acts by footage on the dead soldier's cellphone, showing him "humiliating" a naked woman and her two daughters.
The US news weekly said Hamad described participating in other acts of mutilating regime forces, including militiamen known as shabiha.
"I have another video clip... In the clip I am sawing another shabiha with a saw. The saw we use to cut trees. I sawed him in small pieces and large ones," Time quoted him as saying.
The magazine said Hamad, a Sunni like much of the opposition fighting against President Bashar al-Assad's regime, expressed hatred of members of the leader's Alawite sect.
"Hopefully we will slaughter all of them," he told the magazine.
"They were the ones who killed our children in Baba Amr and raped our women," he said, referring to a neighbourhood of the central city of Homs.
The opposition National Coalition swiftly condemned Hamad's action, saying it "contradicts the morals of the Syrian people, as well as the values and principles of the Free Syrian Army".
US State Department spokesman Patrick Ventrell said Washington had "raised this gruesome act in our recent conversations with leaders of the Supreme Military Council".
They "assured us that they do not support such actions and that this is not representative of the vast majority of the armed opposition," he added.
But UN High Commissioner for Human Rights Navi Pillay demanded a probe into the growing number of allegations of torture, summary executions and other abuses by rebel fighters.
The Free Syrian Army in its statement stressed that it was committed to "freedom and the rule of law for all of the people of Syria whoever they maybe and whatever their religious or ethnic description".