The Syrian government has practiced a policy of "extermination" against thousands of detainees, and is guilty of other crimes against humanity and war crimes, UN-appointed investigators have found.

In a report released Monday, the UN's Commission of Inquiry on Syria determined that "tens of thousands" of people are detained by regime authorities at any one time. "Thousands more have disappeared after initial arrest by State forces or while moving through Government-held territory, or have gone missing after abduction by armed groups," wrote the commission, which was created by the Human Rights Council but operates independently from the UN's human rights office.

Killings outlined in the report were described as taking place with "high frequency, over a long period of time and in multiple locations, with significant logistical support involving vast resources."

"The Government has committed the war crimes of murder, cruel treatment, torture, rape, sexual violence, and outrages upon personal dignity," said investigators," adding that "there are reasonable grounds to believe that the conduct described amounts to extermination as a crime against humanity."

Notably, the commission found that deaths were centrally tracked and that military and civilian superiors have knowledge of crimes, or are at least were aware of allegations and chose to "consciously disregard such information." Illustrating such command responsibility is vital should regime leaders one day find themselves tried for crimes committed during Syria's nearly five-year civil war.

In addition to abuses carried out by the regime, the commission also outlined torture and summary executions perpetrated against prisoners by armed groups including al Qaeda affiliate Nusrah Front and the Islamic State.

The report, entitled "Out of Sight, Out of Mind: Deaths in Detention," covers violations committed between March 2011 — when widespread anti-government protests in Syria broke out — and November 2015, roughly a month after Russia began a bombing campaign in support of the Assad regime. Investigators spoke with 621 survivors and witnesses.

"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses", said Paul Pinheiro, chairman of the commission. "For ordinary Syrians, the spectre of arrest or abduction, and the near-inevitable horrors that follow, have paralysed communities across the country".

In light what they called pervasive and ongoing violations of international law, commission members recommended that the Security Council refer the situation in Syria to the International Criminal Court in the Hague. The commission also implored council members to adopt targeted sanctions against "persons, agencies and groups credibly suspected of being responsible for or complicit in conduct leading to custodial deaths, torture and enforced disappearances." The report, however, did not list those who it believed should be punished.

Neither Syria nor the principal international actors either operating militarily or backing armed groups in the country — including Russia, Iran, the US and Saudi Arabia — have ratified the ICC's Rome Statute; in 2014, permanent Council members Russia and China vetoed a resolution that would have referred Syria to the Court.

Human rights investigators, included those staffing the commission, are routinely denied access to regime controlled areas of Syria. Much of what was previously known about conditions in regime prisons came from a horde of more than 50,000 photographs depicting dead and abused prisoners that were smuggled out by a government defector in 2013. In December 2015, Human Rights Watch was able to identify several victims who exhibited signs of torture, all of whom were detained by regime authorities between May 2011 and August 2013. The systematic photographing of detainees' bodies lends credence to the commission of inquiry's assertions that "high-ranking" members of the Syrian regime are implicated in human rights abuses.