United Nations rights chief Volker Turk on Wednesday said transitional justice was “crucial” for Syria after the fall of Bashar al-Assad, during the first-ever visit by someone in his post to the country.

Since Islamist-led rebels seized Damascus last month, the United Nations has called for Assad and others to be held accountable for the crimes committed during more than 13 years of civil war.

“Transitional justice is crucial as Syria moves forward,” the UN high commissioner for human rights said at a press conference in Damascus.

“Revenge and vengeance are never the answer.”

Syria’s conflict erupted in 2011 after Assad’s brutal crackdown of anti-government protests. More than half a million people were killed and millions displaced from their homes.

Tens of thousands of people have been detained and tortured in the country’s jails, while Assad has been accused of using chemical weapons including banned sarin gas against his own people.

“The enforced disappearances, the torture, the use of chemical weapons, among other atrocity crimes, must be fully investigated,” Turk said.

“And then justice must be served, fairly and impartially,” he added.

Turk said “such acts constitute the most serious crimes under international humanitarian law”.

Among them, “that banned chemicals were used against civilians… and not just once, says a lot about the extreme brutality of the tactics used by the former regime,” Turk said.