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 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".
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.
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 some nine million people to flee their homes.
It 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.