AKIPRESS.COM - Detainees held by the Syrian government are being killed on a massive scale amounting to a state policy of "extermination" of the civilian population, a crime against humanity, United Nations investigators said on Monday, reports Reuters.

The U.N. commission of inquiry called on the Security Council to impose "targeted sanctions" on high-ranking Syrian civilian and military officials responsible for or complicit in deaths, torture and disappearances in custody, but stopped short of naming the suspects.

The independent experts said they had also documented mass executions and torture of prisoners by two jihadi groups, the Nusra Front and Islamic State, also known as ISIS or ISIL. These constituted war crimes and in the case of Islamic State also crimes against humanity, it said.

The report, "Out of Sight, Out of Mind: Deaths in Detention", covers March 10, 2011 to November 30, 2015. It is based on interviews with 621 survivors and witnesses and evidence gathered by the team led by chairman Paulo Pinheiro.

"Over the past four and a half years, thousands of detainees have been killed while in the custody of warring parties," the Commission of Inquiry on Syria said.

The U.N. criticism of the Damascus government comes at a time when its forces have been advancing with the aid of Russian air strikes. A Moscow-backed government assault near the city of Aleppo this month marks one of the biggest momentum shifts in the five year war and helped torpedo peace talks last week.

Pinheiro, noting that the victims were mostly civilian men, told a news briefing: "Never in these five years these facilities that are described in our report have been visited and we have repeatedly asked the government to do so."

There was no immediate response by the government of President Bashar al-Assad, which has rejected previous reports.

"Prison officials, their superiors throughout the hierarchy, high-ranking officials in military hospitals and the military police corps as well as government were aware that deaths on a massive scale were occurring," Pinheiro said.

"Thus we concluded there were 'reasonable grounds' - that is (the threshold) that we apply - to believe that the conduct described amounts to extermination as a crime against humanity."