The United Nations has accused the Islamic State group of committing war crimes including amputations and public executions, sometimes in the presence of children.
The world body also said it believed the Syrian government had used chlorine gas in combating its enemies.
The Sunni fighters, who are bringing weapons from Iraq, have changed the power balance in Syria, consolidating control over large areas and establishing order by imposing Islamic law, the UN said in its latest report, released on Wednesday.
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"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, using the IS group's original name.
"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 US must respect the laws of war and we are concerned about the presence of these children," Paulo Pinheiro, chairman of the UN 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.
The report also says 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.
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 evidential test, they were dropped - the chlorine came with the barrel bombs - dropped from government helicopters particularly onto civilian areas," said Vitit Muntarbhorn, a commissioner.
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 "long-standing findings of systematic torture and deaths of detainees".
The investigators have already drawn up four confidential lists of suspects whom they believe should face international justice, Reuters news agency reported.