UN investigators on Monday, February 8 accused Damascus of "extermination" in its jails and detention centers, saying prisoners were executed, tortured to death or held in such horrific conditions that they perished, AFP reports.

Over the past four-and-a-half years, thousands of detainees have been killed while being held by different sides in Syria's brutal conflict, the UN commission of inquiry on Syria said in its latest report.

The report painted a stark picture of prisons and detention centers run by the Syrian authorities.

"It is apparent that the government authorities administering prisons and detention centers were aware that deaths on a massive scale were occurring," the report said, according to AFP.

It accused Damascus of committing "extermination as a crime against humanity."

The Syrian government was also guilty of committing a range of other “war crimes and crimes against humanity, including murder, rape, torture and enforced disappearance,” the investigators said.

The report, which stretches back to the beginning of the conflict in March 2011 and through last November, is based on 621 interviews, including with more than 200 former detainees who witnessed one or more deaths in custody.