Thousands of civilians are being secretly imprisoned, raped, tortured and exterminated by Syria's government as it wages a bloody civil war, a United Nations commission found Monday.
The U.N. Commission of Inquiry on Syria lifted the lid on what it called was a systematic, country-wide pattern of prisoner abuse by President Bashar Assad's regime — which it said amounted to war crimes and crimes against humanity.
The government's crimes against prisoners included "extermination, murder … torture, imprisonment, enforced disappearance and other inhuman acts," according to a report from the commission published Monday.
Tens of thousands of detainees have been arrested in what the commission described as a "countrywide pattern" of arbitrary detention over allegations such as supporting the opposition or being "insufficiently loyal" to the government.
While most prisoners are men, some women and children as young as seven years old have died in regime custody, the report added.
It called on the U.N. Security Council to impose "targeted sanctions" against individuals "credibly suspected of being responsible" and suggested the Assad regime's actions involving detainees should be classed as war crimes.
One case recounted by the commission involved a prisoner held by the Syrian army having his genitals mutilated during torture and dying three days later.
Another told of an elderly captive in a Homs military prison being hung by his wrists from the ceiling while guards burned his eyes with a cigarette and "pierced his body with a heated, sharp metal object," according to the report. The prisoner died after three hours.
Anti-government forces also are holding captives — mostly government soldiers or fighters from rival groups — "in the most brutal conditions," the report found.
It said ISIS and its fellow Islamist group Jabhat Al-Nusra are guilty of "routinely conducting summary executions" but that most rebel groups were not guilty of "institutionalized or consistent practice of mass arrests ... torture, and killing of detained civilians."
More than 250,000 people have died in Syria's civil war, which has pitted Assad's forces against a patchwork of rebel groups.
The report released Monday — entitled "Out of Sight, Out of Mind: Deaths in Detention in the Syrian Arab Republic" — was based on 621 interviews, including with around 200 former detainees, and extensive documentary material.
The interviews "paint a terrifying picture of the magnitude of the violations taking place," the report stated.
"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses," commission chair Paulo Pinheiro said in an accompanying statement. "For ordinary Syrians, the specter of arrest or abduction, and the near-inevitable horrors that follow, have paralyzed communities across the country."
The commission said that because Syria had refused investigators unfettered access to its territory, it was impossible to know how many detainees had died in custody.
This "systematized violence," the report said, "has taken place out of sight, far from the raging battlefield," adding that "deaths in custody continue to occur in near-total secrecy."
This article originally appeared on NBCNews.com.