Torture, Cruelty Death Damascus Assad troops
A security member loyal to Syria's President Bashar al-Assad gestures to a photographer in DamascusReuters

The Syrian regime has implemented a state policy of extermination against its own population, killing thousands of detainees since civil war broke out in 2011, the UN says. UN human rights investigators found prisoners held by the government of Bashar al-Assad have been subjected to a series of atrocities amounting to war crimes and crimes against humanity.

The report titled Out Of Sight, Out Of Mind: Deaths In Detention also accuses militant groups opposing Damascus, including the Islamic State (Isis) and al-Qaeda affiliate Jabhat al-Nusra, of similar crimes of widespread torture, summary executions and deadly beatings of their captives.

"Detainees held by the government were beaten to death, or died as a result of injuries sustained due to torture. Others perished as a consequence of inhuman living conditions," the paper, based on material evidence as well as interviews with 621 witnesses and survivors, reads.

"The government has committed the crimes against humanity of extermination, murder, rape or other forms of sexual violence, torture, imprisonment, enforced disappearance and other inhuman acts," it adds. "Some anti-government armed groups established makeshift places of detention where captured government soldiers were ill-treated and executed."

Calling for "targeted sanctions", the document says the government is responsible for a wave of arbitrary arrests of purported political opponents that has landed tens of thousands in state facilities at any one time.

Prisoners are tortured, beaten and denied medical care, in many cases resulting in death. Most abuses occur in facilities run by the intelligence services with the knowledge of senior officials tasked with controlling the perpetrators, the report says.

At the General Intelligence Directorate in Kafr Soussa, Damascus, officers directly instructed subordinates on their preferred method of torture.

"Corpses were transported by other prisoners through the corridors, sometimes to be kept in the toilets, before being removed from the branch," the report says. "Evidence obtained indicates that the superiors of the facilities were regularly informed of the deaths of detainees under their control. Prisoners were transferred to military hospitals before they were buried in mass graves."

The findings were released as the Syrian army, backed by Russian air strikes, is said to be gaining ground north of Aleppo.