Tens of thousands of people rounded up by Syrian authorities during a decade of conflict are missing, with many tortured, raped or killed, amounting to war crimes and crimes against humanity, UN investigators said on Monday.
Rebel groups including the Free Syrian Army, Hayat Tahrir al-Sham (HTS) and the Islamic State have also unlawfully detained, tortured and executed civilians in custody, they said.
"The fate of tens of thousands of the victims who were subjected to the arbitrary and incommunicado detention and enforced disappearance by Syrian Government forces, and at a lesser scale, by ISIS, HTS and other armed groups, remains unknown as we reach the end of a decade," the investigators said in their latest report.
The issue of detainees represents a "national trauma" that would affect Syrian society for decades, they said.
The government of Syrian President Bashar al-Assad has denied many previous UN accusations of war crimes and says it does not torture prisoners.
The Commission of Inquiry on Syria, led by Paulo Pinheiro, called for perpetrators on all sides to be prosecuted and for the creation of an international mechanism to locate the missing or their remains, some in mass graves.
The investigators welcomed a court in the German city of Koblenz last week sentencing a former member of Assad's security services to 4-1/2 years in prison for abetting the torture of civilians, the first such verdict for crimes against humanity in the 10-year-old Syrian civil war.
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They have provided information to national jurisdictions handling more than 60 criminal cases and their reports were used as evidence in the Koblenz proceedings, they said last week.
Syria is in the throes of a deep economic crisis amid a collapsing currency and skyrocketing inflation. However, fighting has largely ceased and Assad has taken back control of most of the country thanks to strong Russian military backing.
"Widespread enforced disappearance has been deliberately perpetrated by government security forces throughout the decade on a massive scale, to spread fear, stifle dissent and as a punishment," the report said.
The UN investigators conducted 2,658 interviews, including with some former ex-detainees, and used official documents, photographs, videos and satellite imagery to document crimes in more than 100 detention facilities run by all forces.