United Nations investigators have accused the Syrian Government of "extermination" in its jails and detention centres, saying prisoners have been executed, tortured to death or held in such horrific conditions that they perished.

Key points: Syrian Government policy for detainees amounts to "extermination", UN report says

Syrian Government policy for detainees amounts to "extermination", UN report says UN calls for "targeted sanctions" on officials and for Syria to be referred to the ICC

UN calls for "targeted sanctions" on officials and for Syria to be referred to the ICC The report comes days after failed peace talks

Thousands of detainees have been killed while being held by different sides in Syria's brutal conflict since the violence began nearly five years ago, the UN Commission of Inquiry on Syria said in its latest report.

The report painted a particularly stark picture of prisons and detention centres run by the Syrian authorities, and it said fake death certificates have been issued showing the victims died of natural causes.

"The mass scale of deaths of detainees suggests that the Government of Syria is responsible for acts that amount to extermination as a crime against humanity," commission head Paulo Pinheiro said in Geneva.

The report adds to a huge body of evidence from the commission and others, detailing horrific abuse, torture and killings in Syrian-run jails.

The so-called Caesar Report released in early 2014, contained some 55,000 photographs depicting the tortured and abused bodies of around 11,000 people it said had died in Syrian jails during the first two years of the conflict.

The report, Out of Sight, Out of Mind: Deaths in Detention, 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.

"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses," Mr Pinheiro said.

Detainees suffer 'unimaginable abuses'

The survivors had detailed how their cellmates were beaten to death during interrogation or in their cells, or left to die of severe injuries sustained from gruesome torture.

The more we learn, the more appalling we should find it The report's findings are not surprising unfortunately, but they are damning and they're not easy reading. There are examples given of people being hung from the wrists, of having their eyes burnt out with cigarettes, of having hot metal rods pushed into their bodies and other unspeakable crimes and abuses. But what's important about this report is that it documents a massive series of government institutions which detain people, torture them, kill them and then cover up the deaths with fake death certificates. These investigators call for everyone involved in the conflict in Syria to be referred to the International Criminal Court, that's something the UN Security Council would have to do. The Syrian Government has the cover of Russia for example. And they also point out in this report that the officials who are in charge of these detention centres where these deaths occur know about it and also that the their civilian bosses would know about it. It goes, for more than a year now, right to the top, to President Assad, who was accused of crimes against humanity back then and now down root-and-branch through the security services in his Government. We must remember in the report, some of the more moderate less extreme rebel groups have also been accused of abusing, killing and torturing their prisoners, so just an awful conflict and even though it's not surprising, the more detail we learn of the crimes being committed, the more appalling the world should find it. -Analysis by Middle East correspondent Matt Brown

One witness described how an elderly man held at a military security branch in Homs had been severely beaten and then hung from his wrists from the ceiling.

"The guards burned his eyes with a cigarette and pierced his body with a heated, sharp metal object ... after hanging in the same position for three hours, the man died," the report said.

Others died from lacking medical care and "inhuman living conditions", including severely overcrowded and unhygienic cells and lacking food and clean water, the report said, adding that many prisoners were forced to use their toilet as a source of drinking water.

"A high number of prisoners across detention facilities died of severe and continuing diarrhoea ... victims often suffered for months before death occurred," the report said.

Most of the detainees known to have died are men, but women and children as young as seven have also perished while being held by the Syrian authorities, the report said.

Abuse, squalid conditions and a "high frequency" of deaths were consistent across places of detention and over time, and must have been condoned up the chain of command, it said.

"There are reasonable grounds to believe that high-ranking officers — including the heads of branches and directorates — commanding these detention facilities, those in charge of the military police, as well as their civilian superiors, knew of the vast number of deaths occurring in detention facilities," it said.

"Yet [they] did not take action to prevent abuse, investigate allegations or prosecute those responsible."

They are thus "individually criminally liable", the investigators said, calling again for Syria to be referred to the prosecutor of the International Criminal Court (ICC).

UN slammed for 'doing nothing'

Damascus is not the only one abusing and killing detainees in Syria.

The report detailed horrific abuses carried out in makeshift detention centres run by the Islamic State group, including massacres and executions of children.

Syria Inquiry co-commissioner Carla del Ponte speaks at a news conference. ( Reuters: Pierre Albouy )

The group, notorious for its brutal public executions by beheadings and throwing people off high buildings, has committed war crimes and crimes against humanity, the report said.

Decrying the atmosphere of "total impunity" reigning in Syria, commission member and former ICC prosecutor Carla del Ponte slammed the UN Security Council for "doing nothing".

The commission, which has repeatedly called on the Security Council to refer the situation in Syria to the International Criminal Court, also called for "targeted sanctions" against the people, agencies and groups suspected of being behind the violations, but stopped short of naming them.

Over the past four years, the investigators have drawn up a confidential list of suspected war criminals and units from all sides which is kept in a UN safe in Geneva.

"Accountability for these and other crimes must form part of any political solution," the investigators said, five days after UN-sponsored peace talks were suspended without any result.

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