The United Nations accused Islamic State insurgents on Wednesday of committing war crimes including amputations and public executions, sometimes in the presence of children, and said it believed Damascus had used chlorine gas in combating its enemies.
The Sunni militants, who are bringing weapons from Iraq, have changed the power balance in Syria, consolidating control over large areas and establishing order by imposing harsh sharia law, the UN said in its latest report.
"Executions in public spaces have become a common spectacle on Fridays in al Raqqa and ISIS-controlled areas of Aleppo governorate (province)," the report said.
"Children have been present at the executions, which take the form of beheading or shooting in the head at close range... Bodies are placed on public display, often on crucifixes, for up to three days, serving as a warning to local residents."
The independent investigators voiced deep concern about boys forced to join the ranks of Islamic States who are being trained in camps in Syria that could be targeted by US air strikes.
US President Barack Obama vowed "justice will be done" against the Islamic State killers of American journalist James Foley on Tuesday as the United States sought to identify targets for potential air strikes in Syria.
"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," Paulo Pinheiro, chairman of the U.N. commission of inquiry, told a news briefing in Geneva.
"ISIS poses a clear and present danger to civilians and particularly minorities under its control in Syria and in the region," Pinheiro said.
Syrian government forces have dropped barrel bombs on civilian areas, a war crime under international law, including some believed to contain the poison gas chlorine in eight incidents in April, the investigators said in latest report.
Previously they had only been able to conclude that the chemicals came from Syrian government stockpiles.
"We've looked at eight instances of alleged chlorine attacks and we found specifically that according to our evidentiary test, they were dropped - the chlorine came with the barrel bombs - dropped from government helicopters particularly onto civilian areas," said Vitit Muntarbhorn, a commissioner.
"FRAGMENTED AND VICIOUS"
The conflict, which began with peaceful protests against President Bashar al-Assad in March 2011, has become multi-faceted with more groups involved and many front lines, said Muntarbhorn.
"It's fragmented, it's vicious, and there are cross-border implications," he said.
More than 191,000 people were killed in the first three years of the war, a UN report said last week.
Syria's military and air force continue to commit serious violations, including aerial bombardment and barrel bombing particularly of Aleppo, Deraa and Rural Damascus, Pinheiro said.
"There has been also an increase in reports of enforced disappearances, sexual assaults in detention centres and the arrest of male relatives of wanted persons," he said.
Deaths in custody in Syrian jails are on the rise and forensic analysis of 26,948 photographs allegedly taken from 2011-2013 in government detention centres back its "longstanding findings of systematic torture and deaths of detainees".
"Forced truces, a mark of the government's strategy of siege and bombardment, are often followed by mass arrests of men of fighting age, many of whom disappear," it said.
The UN report, the commission of inquiry's eighth since being set up exactly three years ago, is based on 480 interviews and documentary evidence gathered by its team, which is trying to build a case for future criminal prosecution.
Islamic forces, which are also sweeping through neighbouring Iraq in their bid to establish a cross-border caliphate, have drawn more experienced and ideologically motivated foreign fighters and established control over large areas in northern and eastern Syria, particularly oil-rich Deir al-Zor, it said.
Islamic State is variously referred to as IS, ISIS or ISIL.
The investigators, who include former UN crimes prosecutor Carla del Ponte, have already drawn up four confidential lists of suspects whom they believe should face international justice.
In the report, they reiterated their call for the UN Security Council to refer violations in Syria to the prosecutor of the International Criminal Court (ICC).
"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," del Ponte said.