GENEVA - 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.