The Syrian government has been accused of carrying out a state policy of "extermination" - amounting to a crime against humanity - with detainees dying on a massive scale while held by the Assad regime during the civil war.
A new report by United Nations war crimes investigators said thousands of detainees had been killed over the past four and a half years while in the custody of warring parties in the Syria conflict.
The Commission of Inquiry on Syria also collected evidence of mass executions and torture of prisoners by two jihadi groups, Jabhat al-Nusra and Islamic State (ISIS), which constituted war crimes.
The report said the state-sponsored killings and deaths, which included by starvation, occurred "with high frequency, over a long period of time and in multiple locations" and relied upon "significant logistical support involving vast state resources".
"There are reasonable grounds to believe that the conduct described amounts to extermination as a crime against humanity," the report concluded.
The independent investigators found tens of thousands of detainees are held by President Bashar Assad's government at any one time and said thousands more had "disappeared" after being arrested by state forces or abducted by armed groups.
The report said Syrian military and civilian officials - including "high-ranking officers" - carried out or at least knew of the mass deaths and "of bodies buried anonymously in mass graves" and should therefore be "individually criminally liable".
The investigators urged the UN Security Council to impose "targeted sanctions" on the officials - who were not named in the report - and again called for Syria to be referred to the prosecutor of the International Criminal Court (ICC).
The report meanwhile said the two jihadi groups, Islamic State and Jabhat al-Nusra, had murdered government soldiers and subjected civilians to "illicit trials" by Sharia courts, which ordered death sentences.
"Accountability for these and other crimes must form part of any political solution," the investigators said.
The report, titled "Out of Sight, Out of Mind: Deaths in Detention", covered the period from March 2011 to November 2015.
It was published less than a week after UN-sponsored peace talks were suspended without any result in the conflict.