Human rights investigators said in a report that the Islamic State (IS) had committed crimes against humanity, is forcing young children to join its army, and is lashing women for failing to dress as ordered.
The war crimes report into the three-year conflict in Syria – which has already cost around 200,000 lives – also accused the Syrian government of using chemical weapons in eight separate incidents in April.
The UN investigators said of IS forces: “Bodies of those killed are placed on display for several days, terrorising the local population.
“Women have been lashed for not abiding by IS’s dress code.
“In Raqqa, children as young as ten are being recruited and trained at IS camps.”
The report is based on 480 interviews and documentary material. It cited dozens of documented public executions in Aleppo and Raqqa during the bloody and complex Syrian civil war.
Crowds of people including children have reportedly been made to watch as the group’s fighters pronounce mostly adult men guilty of violating religious laws and then behead them or shoot them in the head at close range. The purpose, according to the commission, is “to instil terror among the population, ensuring submission to its authority”.
Photographs posted online yesterday showed the aftermath of the Islamic State group’s takeover of the Tabqa airbase in Raqqa province.
In one photo, masked gunmen can be seen shooting seven men kneeling in front of them.
Paulo Pinheiro, chairman of the panel, said: “IS poses a clear and present danger to civilians under its control in Syria and in the region.”
Investigators voiced deep concern about boys forced to join the ranks of IS and who are being trained in camps in Syria that could be targeted by American air strikes.
US president Barack Obama vowed on Tuesday that “justice will be done” against the IS killers of American journalist James Foley, as the United States sought to identify targets for potential air strikes in Syria.
Mr Pinheiro, speaking at a news conference in Geneva, said: “We are aware of the presence of children in training camps.
“I think that this decision by the United States must respect the laws of war and we are concerned about the presence of these children.”
In addition to detailing the horrors of the IS militants, investigators also reported on state-sponsored violence which it said had been carried out by government forces loyal to Syrian president Bashar al-Assad.
US investigators said they now believe his forces unleashed a chemical agent – probably chlorine – on civilians in northern Syrian villages.
Victims and medical workers described symptoms caused by exposure to chemicals and witnesses told of a chlorine-like smell immediately after government helicopters bombed civilian areas in Idlib and Hama provinces eight times between 11 and 29 April.
Vitit Muntarbhorn, a law professor and member of the UN commission, said: “There are reasonable grounds to believe that the chlorine has been dropped, particularly in barrel bombs from helicopters belonging to the government authorities. So the finger points there.”
The report said attacks have taken place in the northern province of Aleppo and in the north-eastern region of Raqqa, a stronghold of the Islamic State.
The commission also emphasised that Syrian regime forces continue to perpetrate crimes against humanity through massacres and systematic murder, torture, rape and disappearances.
It is also said that other factions fighting Mr Assad’s government were committing massacres and war crimes.
Mr Pinheiro said the international community has failed “in its most elemental duties; to protect civilians, halt and prevent atrocities and create a path toward accountability”.
One of the investigators, Carla del Ponte, a former chief prosecutor of the UN war crimes tribunals in the former Yugoslavia and Rwanda, yesterday urged the world’s leading powers to refer Syria to the International Criminal Court.
The former Swiss attorney-general also said investigators had already drawn up four lists of suspects they believe should face international justice.
“We still hope that by holding all this evidence in our archives, one day a prosecutor’s office can use it to conduct a formal inquiry and prepare indictments,” Ms del Ponte said.