The United Nations on Wednesday accused both the Islamic State (IS) and the Syrian government of committing war crimes.
A 45-page report issued in Geneva accused IS of murder, torture, enforced disappearnaces and forced displacement against the civilian population in Syria.
"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 report stated.
The report also accused the government of bombing civilian areas, possibly using chemical agents, as well as torturing and killing prisoners.
Syria's military is now shifting its focus of operations against the Islamic state, Reuters reported on Wednesday.
IS fighters reportedly control one-third of the country in the north and east of Syria. They captured a major military air base in northeastern Syria on Sunday.
Syrian army officers said that recent gains against armed rebels, which have been fighting to overthrow the government of Bashar Assad since 2011, will allow it to focus fighting against the extremist insurgency which is considered a more serious threat.
"From a military point of view, IS is one of the biggest dangers for Assad," Jochen Hippler, political scientist at the University of Duisburg in western Germany, told German public radio.
Reuters also reported on Wednesday that IS fighters have executed Syrian soldiers and are holding a group of others prisoner.
"Many Syrians don't even perceive Assad as the main threat anymore, he is now the lesser of two evils," Hippler added.