"Any political transition must ensure accountability for perpetrators of serious violations and guarantee that those responsible are held to account," he told reporters in Geneva.

Asked about whether Assad was among those who should be brought to account, Turk stressed the need "for fair trials in relation to all those where indeed there are serious grounds to believe that they have committed atrocity crimes".

"And that goes also for the... former president of Syria and whoever was in senior leadership positions."

Turk insisted it was "imperative that all evidence be collected and preserved meticulously for future use".

Turk said he would "strongly encourage" the new Syrian authorities to ratify the statute of the International Criminal Court, paving the way for perpetrators to be held to account there, stressing also the need to "build up a domestic legal system that allows for fair trials".

His comments came after Assad fled Syria as Islamist-led rebels swept into Damascus, triggering celebrations across the country and beyond at the end of his oppressive rule.

Assad's government fell in just 11 days after the rebels began a surprise advance, more than 13 years after Assad's crackdown on anti-government protests ignited Syria's civil war -- which had become largely dormant until the rebel push.

Syria's war, which erupted in 2011, has killed more than 500,000 people and forced half of the population to flee their homes.

Turk said that "the most serious human rights violations" included "torture and the use of chemical weapons".

The sudden and dramatic shift in Syria brought "hope that this would be an opportunity for the country to build a future grounded in human rights, freedom and justice", Turk said.

He added that "reform of the security apparatus will be key".

Turk said the new authorities would need t address the "tragedy" of the more than 100,000 people who had disappeared during the civil war.

With hostilities still reportedly ongoing in parts of Syria, Turk said it was "imperative that all parties abide by their obligations under international humanitarian law and human rights law".

In particular, he said, "all measures must be taken to ensure the protection of all minorities, and to avert reprisals and acts of revenge".

Turk insisted that "the only way forward is a nationally owned political process that brings an end to the litany of suffering, fulfils the aspirations of all Syrians, and ensures truth, justice, reparation, healing and reconciliation".

There is a need, he said, for "an internal process" bringing together "all the actors", including armed groups, and existing state institutions, but also "an external process" focusing also on those "impinging on the sovereignty of the country".