Syrian rebel authorities have vowed to punish a fighter who was shown in a gruesome video apparently cutting out and eating the internal organs of a government soldier.
The incident has highlighted war crimes allegations against the opposition and put the rebel leadership and its backers on the defensive.
"Any act contrary to the values that the Syrian people have paid their blood and lost their homes [for] will not be tolerated, the abuser will be punished severely even if they are associated with the Free Syrian Army," rebel leaders said.
It said field commanders had been instructed "to begin a prompt investigation into the matter in which the perpetrator will be brought to justice".
The rebel in the video has defended his actions as revenge for regime atrocities, Time magazine reported on Tuesday.
United States government officials said it was the act of an individual unrepresentative of the armed opposition as a whole but the UN human rights chief demanded an investigation into a growing number of allegations of "very serious violations by opposition fighters".
Time said it had talked with the fighter in the video, It identified him as Khalid al-Hamad.
Hamad claimed he was driven by footage on the dead soldier's mobile phone, showing him "humiliating" a naked woman and her two daughters.
Time said Hamad described participating in other acts of mutilating regime forces, including militiamen known as shabiha.
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 the city of Homs.
The video, in which Hamad leans over a uniformed body, cuts out organs and then holds one up to his mouth, has prompted an outcry around the world.
The opposition National Coalition swiftly condemned the action, saying it "contradicts the morals of the Syrian people, as well as the values and principles of the Free Syrian Army".
A US State Department official 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 said.
But UN High Commissioner for Human Rights Navi Pillay demanded investigations into the growing allegations of torture, summary executions and other abuses by rebel fighters.
The Britain-based Syrian Observatory for Human Rights reported on Tuesday that three captured army officers had been executed in Raqa by fighters of the Al-Nusra Front.
The group, which has pledged loyalty to al-Qaeda, is not part of the Free Syrian Army but is a major fighting force.
Pillay described the video as a "truly atrocious act" and asked rebel commanders to "do everything in their power to halt such gross crimes".
"They must investigate this incident along with other alleged very serious violations by opposition fighters, including acts of torture and a succession of apparent summary executions and extra-judicial killings," she said.