The United Nations says an inquiry has produced evidence that war crimes were authorised in Syria at the highest level.
It is the first time the UN's human rights agency has so directly implicated President Bashar al-Assad.
"Massive evidence ... (of) very serious crimes, war crimes, crimes against humanity," said Commissioner Navi Pillay in Geneva.
"The scale of viciousness of the abuses being perpetrated by elements on both sides almost defies belief.''
Ms Pillay said the UN commission of inquiry had compiled a list of those believed to be directly responsible for serious human rights violations.
However, the BBC reports the names and specific evidence relating to them remain confidential pending a possible prosecution for war crimes and crimes against humanity by the International Criminal Court.
Ms Pillay has previously called on the UN Security Council to refer Syria to the ICC.
But Syria is not a state party to the ICC and therefore any investigation into the conflict would need to be mandated by the UN Security Council.
However, Russia and China have a veto on the council and the BBC reprots they would be highly unlikely to let such a move pass.
The inquiry has also previously reported it has evidence that rebel forces in Syria have been guilty of human rights abuses.