The Assad regime is killing so many detainees in Syria that it now amounts to the crime against humanity of "extermination", a UN report has found.
In a document published by the UN's High Commissioner for Human Rights, investigators found the Syrian government responsible for "massive and systematised violence".
The crimes against humanity committed by the Assad regime, according to the UN, far outnumber those of Isis militants and other jihadist groups.
The UN commission of inquiry called on the Security Council to imporse "targeted sanctions" against Syrian officials. Its report was based on interviews with 621 survivors and witnesses and covers the period March 2011 to November 2015.
"The situation of detainees in Syria is critical, and represents an urgent and largescale crisis of human rights protection," the Commission of Inquiry in Syria report said.
"With thousands of persons still in custody, urgent steps need to be taken by the Syrian Government, armed groups, the external backers of various belligerents, and the wider international community to prevent further deaths."
It said that in the interviews it conducted, almost every one of over 500 survivors of government detention centres said they were victims of "torture and inhuman and degrading treatment".
Some witnessed others being beaten to death during interrogations or in their cells, while others died "as a consequence of inhuman living conditions inflicted on the prison population".
According to the UN, the regime of Bashar al-Assad has committed "the crimes against humanity of extermination; murder; rape or other forms of sexual violence; torture; imprisonment; enforced disappearance and other inhuman acts". It said war crimes were also committed "based on the same conduct".
By comparison, jihadist groups such as Isis and Jabhat al-Nusra were accused of the crimes against humanity of murder and torture, as well as war crimes.
"Accountability for these and other crimes must form part of any political solution," the report said.
The commission of inquiry found there are "reasonable grounds to believe that high ranking officers... knew of the vast number of deaths occurring in detention facilities under their control".
It said such people, in the heighest ranks of the Syrian regime, were "individually criminally liable" - but did not go so far as to name names.
Among its recommendations, the commission suggested the UN Security Council demand all sides end custodial deaths and torture and answer to the International Criminal Court.
And it called on the Security Council to "adopt targeted sacntions against persons, agencies and groups credibly suspected of being responsible for or complicit in conduct leading to custodial deaths, torture and enforced disappearances".
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