The Free Syrian Army says any rebel fighter who carries out atrocities will be punished.
The comment was in response to a video showing a rebel commander cutting out and biting into the organs of a dead government soldier.
The gruesome video has been a setback for the rebels, as the United States and other Western powers contemplate giving them direct military aid.
In an interview with TIME magazine, the man in the video expressed hatred of all members of the Alawite community, the religious minority to which President Bashar al-Assad belongs.
"Hopefully we will slaughter all of them," he told the magazine.
"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," the main rebel group said in a statement.
The Syrian Observatory for Human Rights in London reported on Tuesday that three captured army officers had been summarily executed in the northern city of Raqa by fighters of the Al-Nusra Front.