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, United Nations investigators said Friday.

Their report, based on more than 300 interviews with witnesses and victims, called on world powers to bring the commanders before the International Criminal Court for both war crimes and crimes against humanity.

"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 have swollen the group's ranks and dominate its leadership structure, the report said. A separate UN report has said 15,000 foreigners have gone to fight in Syria and Iraq.

"The commanders of ISIS have acted willfully, perpetrating these war crimes.... They are individually criminally responsible," the report added, saying the group's leader, Abu Bakr al-Baghdadi, wielded "absolute power."

Paulo Pinheiro, who chaired the UN panel, said at a news conference that its findings would add to a secret list of war crimes suspects from all sides in Syria's civil war, in which some 200,000 people have been killed since March 2011.

Since US-led air strikes began targeting IS forces in Syria in late September, its fighters have begun taking up positions in civilian houses and farms, leading to civilian casualties, the report said.

It found that the group, also known as ISIS, was depriving 600,000 people in the north of deliveries of food and medical aid, and enforcing its radical interpretation of Islamic law through "morality police."

These ordered lashings and amputations for offenses such as smoking cigarettes or theft; one female dentist in Deir al-Zor had been beheaded for treating patients of both sexes.

"ISIS has beheaded, shot and stoned men, women and children in public spaces in towns and villages across northeastern Syria," the report said.

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Children were being pressed to inform on their parents, women stoned for unapproved contact with men, and Christians, Kurds and other minorities forced to convert to Islam or pay a tax: "Witnesses saw scenes of still-bleeding bodies hanging from crosses, and of heads placed on spikes along park railings."

Among those slaughtered in Syria were 200 soldiers captured from the Tabqa airbase in Raqqa province, and hundreds of members of the al-Sheitat tribe in Deir al-Zor.