UN Secretary General Antonio Guterres called for justice for victims of torture and other atrocities in Syria's decade-long war, saying civilians had suffered the "greatest crimes the world has witnessed this century".
Speaking before the 10th anniversary of the start of anti-government protests that spiralled into civil war, Mr Guterres denounced widespread torture, illegal detention and chemical weapons attacks in Syria.
Last month there was a landmark prosecution of a former Syrian government secret police officer by a court in Germany, but there are concerns that such trials are rare and that many war crimes will go unpunished.
“It is impossible to fully fathom the extent of the devastation in Syria but its people have endured some of the greatest crimes the world has witnessed this century,” Mr Guterres said in New York on Wednesday.
“The scale of the atrocities shocks the conscience. Their perpetrators must be held to account if there is to be sustainable peace in Syria.”
Mr Guterres criticised forces loyal to the government of Syrian President Bashar Al Assad, foreign forces involved in the conflict and terrorist militias that have “subjected many Syrians to unimaginable violence and repression”.
The warring forces devastated towns and villages, bombed schools and hospitals, blocked aid flows, and used nerve gas and other deadly chemical weapons in atrocities that flout international law, he said.
“The situation remains a living nightmare,” Mr Guterres said. “Hundreds of thousands of Syrians have died. Millions have been displaced.
"Countless others remain illegally detained and often tortured, missing, disappeared or living in uncertainty and deprivation.”
Millions of people left Syria and millions more have fled their homes since a crackdown by the government on protesters in March 2011 led to a civil war that has dragged in Russia, Iran, Turkey, the US and others.
A court in Germany last month sentenced former Syrian secret police officer Eyad Al Gharib to four and a half years in prison for aiding and abetting crimes against humanity in the detention and torture of anti-government protesters.
Rights groups hailed the first successful prosecution of a former Syrian regime official for crimes against humanity, and the use of universal jurisdiction by German prosecutors, but also noted that most Syrian war criminals have eluded justice.