GENEVA (AP) — International investigators say several thousands of detainees 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 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-Qaida-affiliated Nusra Front.

The report is drawn from 621 interviews conducted between March 2011 and November 2015. Investigators say they are short of enough evidence to provide more specific estimates of killings of those detained.

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.