The jihadist Islamic State of Iraq and the Levant (ISIL) carried out "mass executions" in Syria in January, the United Nations said Tuesday, adding that it was investigating allegations of mass graves.
The group "conducted mass executions of detainees, thereby perpetrating war crimes," the UN Commission of Inquiry on Syria said in an update on the human rights situation in the country.
ISIL is a group of radical jihadists who have fought Syria's regime but who since the beginning of the year have also been increasingly battling a coalition of moderate and Islamist rebels angered by their abuses of rival fighters and civilians.
The group is accused of kidnapping scores of people, among them rival rebels, activists, foreign journalists and humanitarian workers.
In northern and northeastern governorates, ISIL fighters "are imposing their radical ideologies on the civilian population," Paulo Sergio Pinheiro, head of the commission, told the UN Human Rights Council.
Tuesday's update followed a report published earlier this month in which the commission said ISIL already last year had committed "crimes against humanity" in Raqqa province by systematically subjecting civilians to "severe physical or mental pain or suffering".
- 'Execution fields' -
Then in January, as hostilities between the different armed groups escalated, ISIL killed prisoners including civilians on "execution fields", the commission said Tuesday.
It said it had documented a number of mass executions in the Idlib, Aleppo and Raqqa governorates, including in a children's hospital in Aleppo that the group was using as its headquarters.
ISIL had also executed captured members of the Al-Qaeda-affiliated Al-Nusra Front and others in Tel Abyad in Raqqa province, with witnesses describing dead bodies with their "hands tied behind their backs with strips of cloth and a single shot to the head," the commission said.
It said the executions appeared to have been carried out in anticipation of a military loss and that "many killings occurred hastily, at pointblank range."
"The number killed as well as allegations of mass graves connected to these executions remain under investigation," it said.
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The commission was created three years ago by the UN Human Rights Council to investigate abuses committed in the war, which is estimated to have claimed some 146,000 lives and forced millions to flee their homes.
In its report earlier this month, the panel highlighted the plight of more than 250,000 people in areas besieged by both sides in the conflict.
"Civilians in besieged areas have been reduced to scavenging... People, including young children, have starved to death," Pinheiro lamented Tuesday.
- Confidential lists of perpetrators -
The commission has never gained access to Syria, relying on more than 2,700 interviews conducted in the region and from Geneva for its findings.
It has drawn up four confidential lists of people and groups on both sides of the conflict whom it believes should be held accountable for a litany of war crimes and crimes against humanity.
Pinheiro refused to provide reporters with names Tuesday, but said they included "persons criminally responsible for hostage-taking, torture and executions".
"The heads of intelligence branches and detention facilities where detainees are tortured, names of military commanders who target civilians, airports from which barrel bomb attacks are planned and executed and armed groups involved in attacking and displacing civilians" also figured on the lists, he said.
Commission member Carla del Ponte meanwhile told reporters that "referral to justice is an urgent, an extremely urgent need."
The commission has long called for the crimes to be referred to the International Criminal Court in the Hague, but for that to happen, a blocked UN Security Council would need to give the green light.
Del Ponte, a legendary former war crimes prosecutor, said she thought it would be preferable for perpetrators in the Syrian conflict to be tried by an ad hoc tribunal similar to the ones created for the former Yugoslavia and Rwanda.
Del Ponte, who served as prosecutor for both of those tribunals, said she would like to serve on a possible court for Syria too, stressing the commission's "good work" in collecting "a lot of evidence that can be used to prepare an indictment".
Ad hoc tribunals also however depend on the Security Council's approval.