GENEVA (Reuters) - An estimated 3,500 people, mainly women and children, are being held as slaves in Iraq by Islamic State militants, the United Nations said on Tuesday.

Yazidi sisters, who escaped from captivity by Islamic State (IS) militants, sit in a tent at Sharya refugee camp on the outskirts of Duhok province July 3, 2015. REUTERS/Ari Jala

The Islamist group, which also controls large parts of Syria, is responsible for acts that may “amount to war crimes, crimes against humanity, and possibly genocide”, particularly against minorities, a report said.

Iraqi security forces and allied groups including Kurdish Peshmerga fighters have also killed and abducted civilians, it said. “Some of these incidents may have been reprisals against persons perceived to support or be associated with ISIL (Islamic State),” it added.

At least 18,802 civilians were killed in violence in Iraq from January 2014 to October 2015, and 36,245 civilians were wounded, the report said, calling the figures “obscene”.

The U.N. Assistance Mission for Iraq and the U.N. human rights office estimated that 3,500 people were “currently being held in slavery” by Islamic State, which seized mainly Sunni-populated areas in the north and west in 2014.

“Those being held are predominantly women and children and come primarily from the Yazidi community,” said the joint report issued in Geneva, referring to a non-Muslim minority in northern Iraq viewed by Islamic State as devil-worshippers.

“But a number are also from other ethnic and religious minority communities.”

The Sunni Islamists, who claim responsibility for suicide bombings in Baghdad against Shia mosques and markets, should face prosecution for international crimes, said Francesco Motta, director of the U.N. human rights office in Iraq.

“They use civilians as shields. They use children in armed conflict, they also directly target civilian infrastructure and that can amount to war crimes but they can also constitute crimes against humanity,” he told a news briefing by telephone from Baghdad.

The group seeks to “basically eliminate, purge or destroy minority communities”, Motta said.

“We’ve seen communities like the Yazidi in particular bear the brunt of this. Yazidi were basically given the option by ISIL to convert or to be killed.

“The intent seems clear ... the international crime of genocide,” he added. “The intention was to destroy part or the whole of the Yazidi people.”

The report detailed Islamic State executions by shooting, beheading, bulldozing, burning alive and throwing people off buildings. Doctors, teachers and journalists opposed to its ideology have been “singled out and murdered by ISIL”.

“We have a lot of information on the recruitment of children, children as young as nine, to train them sometimes to use them as suicide operatives in their operations, but also forcing them to give blood and also take armed combat roles in other parts where conflict is taking place,” Motta said.

Between 800 and 900 children in Mosul had been abducted for military and religious training, the report said.

Ramadi, capital of Anbar province, was recaptured from Islamic State in late December, and the tide of fighting appears to have turned against the group.

“We still have grave fears for civilians in areas under Daesh (Islamic State) control as the armed forces and those supporting the government move closer to those areas,” Motta said.

(Corrects timeframe for casualty figures in paragraph four)