United Nations investigators say thousands of detainees have been executed, beaten to death or otherwise left to die as part of an “extermination” during Syria's civil war, in policies that appear to amount to crimes against humanity.

The U.N.-backed Commission of Inquiry on Syria presented a 25-page report Monday on killings of detainees by President Bashar Assad's government. It also cites execution policies by radical groups like the Islamic State and Al Qaeda-affiliated Nusra Front.

"The killings and deaths described in this report occurred with high frequency, over a long period of time and in multiple locations, with significant logistical support involving vast State resources," the report said, according to Reuters. "There are reasonable grounds to believe that the conduct described amounts to extermination as a crime against humanity."

The report is drawn from 621 interviews conducted between March 2011 and November 2015. It stated that “over the past four and a half years, thousands of detainees have been killed while in the custody of warring parties.”

The report seeks "targeted sanctions" against unspecified individuals or groups responsible for such crimes. The investigators lamented inaction by the U.N. Security Council about possibly launching criminal probes.

According to the report, there are reasonable grounds to believe that “high-ranking officers” within Assad’s government knew of the killings and burials of bodies, and are thus “individually criminally liable.”

The Associated Press contributed to this report.