The Free Syrian Army has vowed 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 main rebel group said in a statement.
"The abuser will be punished severely even if they are associated with 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.