The UN's human rights chief has said an inquiry has produced evidence that war crimes were authorised in Syria at the "highest level", including by President Bashar al-Assad.
It is the first time the UN's human rights office has so directly implicated Mr Assad.
Commissioner Navi Pillay said her office held a list of others implicated by the inquiry.
The UN estimates more than 100,000 people have died in the conflict.
The UN's commission of inquiry into Syria has produced "massive evidence... [of] very serious crimes, war crimes, crimes against humanity," Ms Pillay said.
"The scale of viciousness of the abuses being perpetrated by elements on both sides almost defies belief," she said.
The evidence indicated responsibility "at the highest level of government, including the head of state", she added.
The inquiry has also previously reported it has evidence that rebel forces in Syria have been guilty of human rights abuses.
However, the investigators have always said the Syrian government appears to be responsible for the majority, and that the systematic nature of the abuse points to government policy.
Syrian Deputy Foreign Minister Faisal Mekdad was dismissive of Ms Pillay's remarks.
"She has been talking nonsense for a long time and we don't listen to her," he told AP.
Mr Mekdad was in The Hague at a meeting of the Organization for the Prohibition of Chemical Weapons (OPCW) to discuss the effort to destroy Syria's chemical weapons.
He told the BBC that Syria needed more money and equipment from the international community.
He said Syria needed lorries and armoured vehicles to transport chemicals to prevent "terrorists" attacking the vehicles on their way to the port of Latakia, where they will be loaded onto a US naval vessel for destruction.
An OPCW spokesman at the conference told the BBC that any donations of dual-use equipment would be carefully monitored and there would have to be strict guidelines imposed to make sure the machinery could only be used for the purpose of removing the weapons.
Death toll 'over 125,000'
Ms Pillay said the UN commission of inquiry had compiled a list of those believed to be directly responsible for serious human rights violations.
It is assumed that senior figures in the Syrian military and government are on that list, the BBC's Imogen Foulkes reports from Geneva.
However, 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 (ICC).
She has previously called on the UN Security Council to refer Syria to the ICC.
Syria is not a state party to the ICC and therefore any investigation into the conflict would need to be mandated by the Security Council.
However, Russia and China have a veto on the council and would be highly unlikely to let such a move pass.
Ms Pillay's statement is a reminder of the severity of the situation in Syria as preparations are made for the Geneva II peace conference next month, our correspondent says.
Both the government and the opposition National Coalition have said they will attend the conference, but the head of the Western-backed rebel Free Syrian Army has said it will continue fighting during the talks.
The National Coalition says it categorically rejects any role for President Assad in any transitional government, while the regime has said it is not going to negotiate a "handover of power".
Also on Monday, the Syrian Observatory for Human Rights (SOHR), a UK-based activist group which monitors deaths in the conflict, said its estimate of the number of dead had now reached 125,835, more than a third of them civilians.
Almost 28,000 rebel fighters had died, and more than 50,000 on the side of the government, including both regular soldiers and pro-regime militias. The latter figure also includes almost 500 dead from the Lebanese Hezbollah movement and other foreign Shia militias.
However, it said it believed these figures were an underestimate as both sides were reticent about reporting deaths in their ranks.
The SOHR also reported on Monday that Islamist rebels had seized control of the town of Maaloula, which houses a historic Christian community.
Earlier the state-run Sana news agency had claimed that fighters from Islamist brigades had stormed Maaloula's St Tekla monastery and had detained some of the nuns that live there.
However, information is difficulty to verify as access for foreign journalists is restricted.
After fighting in the town in September, a nun at the monastery told the BBC she did not think the towns Christians had been deliberately targeted by rebels.