The United Nations' High Commissioner for Human Rights Navi Pillay has declared that war crimes and crimes against humanity were authorised in Syria at the "highest level", including by President Bashar al-Assad.
A UN commission regarding human rights violations in Syria "has produced massive evidence of very serious crimes, war crimes, crimes against humanity," Pillay said.
"The evidence indicates responsibility at the highest level of government, including the head of state," she added.
"The scale and viciousness of the abuses being perpetrated by elements on both sides almost defies belief."
It was the first time that Assad had been held directly responsible for atrocities in Syria by the UN human rights office. Rebel forces had been implicated by the office in human rights abuses.
The Syrian civil war has seen more than 120,000 people killed and two million Syrians displaced. The estimated death toll of the conflict was raised to 125,835 by the Syrian Observatory for Human Rights, a UK-based activist group.
Syria's deputy foreign minister Faisal Mekdad disregarded Pillay's remarks.
"She has been talking nonsense for a long time and we don't listen to her," he said.
Pillay added that the UN commission had created a list of figures believed to be directly accountable for serious human rights violations in the country.
The actual charges and the evidence against them will remain confidential until a prosecution for war crimes is brought before the International Criminal Court (ICC).
Pillay has called on the UN Security Council in the past to take Syria to the court but it is unlikely that such a move would succeed as in the face of Russian and Chinese opposition.
Of the total number of deaths in the conflict, 28,000 were rebel fighters, 50,000 were linked to the government and more than a third were civilians.