Detainees held by the Syrian government are dying on a massive scale amounting to a state policy of "extermination" of the civilian population, a crime against humanity, United Nations investigators said on Monday.

The UN commission of inquiry called on the Security Council to impose "targeted sanctions" on Syrian officials in the civilian and military hierarchy responsible for or complicit in deaths, torture and disappearances in custody, but stopped short of naming them.

In their report, the independent experts said they had also documented mass executions and torture of prisoners by two groups, Jabhat al-Nusra and ISIS, constituting war crimes.

The report, "Out of Sight, Out of Mind: Deaths in Detention", covers March 10, 2011 to November 30, 2015. It is based on interviews with 621 survivors and witnesses and evidence gathered by the team led by chairman Paulo Pinheiro.

"Over the past four and a half years, thousands of detainees have been killed while in the custody of warring parties," the Commission of Inquiry on Syria said. "The killings and deaths described in this report occurred with high frequency, over a long period of time and in multiple locations, with significant logistical support involving vast State resources," the report said.

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Syrians gather on Sunday at a temporary refugee camp for displaced Syrians in northern Syria, near Bab al-Salameh border crossing with Turkey. ( Associated Press)

"There are reasonable grounds to believe that the conduct described amounts to extermination as a crime against humanity."

Mass arrests, starvation

Tens of thousands of detainees are held by the government of Syrian President Bashar al-Assad at any one time, and thousands more have "disappeared" after arrest by state forces or gone missing after abduction by armed groups, it said.

Through mass arrests and killing of civilians, including by starvation and untreated wounds and disease, state forces have "engaged in the multiple commissions of crimes, amounting to a systematic and widespread attack against a civilian population".

There were reasonable grounds to believe that "high-ranking officers", including the heads of branches and directorates commanding the detention facilities and military police, as well as their civilian superiors, knew of the deaths and of bodies buried anonymously in mass graves.

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).

Over the past four years, the investigators, who include former ICC prosecutor Carla del Ponte, have drawn up a confidential list of suspected war criminals and units from all sides which is kept in a UN safe in Geneva.

Jabhat al-Nusra and ISIS have committed mass executions of captured government soldiers and subjected civilians to "illicit trials" by Sharia courts which ordered death sentences, the report said.

"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.