International investigators have said that several thousand prisoners have been executed, beaten to death or otherwise left to die during Syria's civil war, in policies that appear to amount to "extermination" under international law.
The UN-backed Commission of Inquiry on Syria presented a 25-page report on Monday on killings of detainees by President Bashar al-Assad's government.
The report is drawn from 621 interviews conducted between March 2011 and November 2015.
Investigators say that they are short of enough evidence to provide more specific estimates of killings of those in custody.
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The report seeks "targeted sanctions" against unspecified individuals or groups responsible for such crimes.
The investigators lamented inaction by the UN Security Council about possibly launching criminal investigations.
The findings painted a stark picture of prisons and detention centres run by the Syrian authorities.
"The mass scale of deaths of detainees suggests that the government of Syria is responsible for acts that amount to extermination as a crime against humanity," Paulo Pinheiro, the commission's head, said.
Pinheiro described how people held in governmental detention centres were "subjected to violations on a mass scale".
He said "prisoners are routinely tortured and beaten, forced to live in unsanitary and overcrowded cells, [and] with little food and no medical care many perish".
"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses," he said.
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The survivors had detailed how their cellmates were beaten to death during interrogation or in their cell, left to die from severe injuries sustained from torture or from unattended medical conditions, the report said.
It also cites execution policies by armed groups such as the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front.
The report detailed horrific abuses carried out in makeshift detention centres run by ISIL, including massacres and executions of children.
The group, known for its public executions by beheading and throwing people off high buildings, has committed war crimes and crimes against humanity, the report said.
The report also addressed the abuse and killing of detainees by other armed opposition groups, including al-Nusra Front, which it accused of war crimes for torturing and summarily executing mainly captured government soldiers.
Decrying the atmosphere of "total impunity" reigning in Syria, Carla del Ponte, a commission member, criticised the UN Security Council for "doing nothing".
The Syrian conflict started as a largely unarmed uprising against the Assad government in March 2011.
It later morphed into a full-blown civil war that has killed more than 250,000 people and turned more than 4.3 million others into refugees, according to statistics by the UN.
Source: Al Jazeera and agencies