The United Nations human rights chief on Thursday urged countries to step up trials in their national courts against suspected war criminals in Syria as the conflict marks its 10th anniversary.
Attempts to refer atrocities in Syria to the International Criminal Court (ICC) in the Hague for prosecution have failed, Michelle Bachelet said.
Only one person has been convicted in a foreign court for crimes against humanity or war crimes in a conflict that has cost hundreds of thousands of lives, many of them civilians.
Bachelet also called for increased efforts to trace tens of thousands of missing, whom she said included people held in jails run by government forces across Syria.
Pro-democracy protests against President Bashar al-Assad's authoritarian family rule erupted in March 2011 in southern Syria and were met by a crackdown. But they spread quickly across the country and developed into a multi-sided war that in addition to the numerous casualties has forced more than 11 million from their homes, around half the pre-war population.
Assad's forces, backed by Iran and Russia, have recaptured many areas once held by rebel factions and Islamist militants, but the war grinds on in some corners of Syria.
"We owe it to the victims to ensure the next decade is one of accountability and remedy, with their rights and needs addressed so they can rebuild their lives," Bachelet said in a statement.
It is thought that many possible war criminals have relocated overseas.
A German court sentenced a former member of President Bashar al-Assad's security services to 4-1/2 years in prison last month for abetting the torture of civilians, the first such verdict for crimes against humanity in the Syrian war.
"It remains vital that national courts continue to conduct fair, public and transparent trials and reduce the accountability gap for such serious crimes," Bachelet said, calling the conviction "an important step forward on the path to justice".
Assad's government has denied many previous UN accusations of war crimes and says it does not torture prisoners.
UN panel member Hanny Megally said it had been approached by 60 jurisdictions seeking information and it had provided information in about 300 ongoing cases.
The International Committee of the Red Cross (ICRC) visits prisoners in central prisons in Syria but does not have access to unofficial sites. Its findings are confidential.
"Clearly detention is one of these central issues and every day we try to first, improve the conditions of detention and two, enlarge our access," Fabrizio Carboni, ICRC regional director for the Middle East, told Reuters on Tuesday.