By Stephanie Nebehay
GENEVA (Reuters) - Islamic State commanders are liable for war crimes on a "massive scale" in northeast Syria where they spread terror by beheading, stoning and shooting civilians and captured fighters, U.N. investigators said on Friday.
The experts told world powers to make sure the commanders guilty of war crimes and crimes against humanity were held accountable by the International Criminal Court.
The latest report by the independent U.N. investigators is based on interviews with more than 300 men, women and children who fled or still live in Islamic State's northeastern stronghold, including Aleppo.
"In carrying out mass killings of captured fighters and civilians following military assaults, ISIS (Islamic State) members have perpetrated egregious violations of binding international humanitarian law and the war crime of murder on a massive scale," said the report.
Foreign fighters, many of them recruited by violent videos, have swollen the group's ranks and dominate its leadership structure, the report said.
"The commanders of ISIS have acted wilfully, perpetrating these war crimes ... They are individually criminally responsible," it added, saying the group's leader Abu Bakr Al-Baghdadi wielded "absolute power".
Since U.S.-led air strikes began targeting Islamic State in Syria in late September, "civilians living in Minbij (Aleppo) described how ISIS fighters began to position themselves in civilian houses and farms," the 20-page U.N. report said.
"Air strikes on ISIS positions have led to some civilian casualties," it said, without giving an estimate.
Around 200,000 people have been killed in the conflict, the United Nations says.
Islamic State enforces its edicts, based on its radical interpretation of Islamic law, through al-Hisbah "morality police" who order lashings and amputations "for offences such as smoking cigarettes or theft", the U.N. report said.
Children are pressed to inform on their parents, women are executed for unapproved contact with men, and Christians and other minorities are forced to pay taxes or convert, it said.
"ISIS has beheaded, shot and stoned men, women and children in public spaces in towns and villages across northeastern Syria," it said.
"The mutilated bodies of male victims are often placed on display, a warning to the local population of the consequences of failure to submit to the armed group's authority," it added.
Executions have been recorded in Aleppo, Raqqa, Idlib, Al-Hassakeh and Deir Al-Zor provinces, the investigators said.
"Witnesses saw scenes of still-bleeding bodies hanging from crosses and of heads placed on spikes along park railings," the experts added.
(Reporting by Stephanie Nebehay; Editing by Andrew Heavens)