NEW YORK (NYTIMES) - Islamic State in Iraq and Syria (ISIS) fighters executed at least 741 Iraqi civilians in Mosul, including women and children who had tried to flee, during the nine-month battle by government forces to retake the northern Iraq city from the militant group, the United Nations said on Thursday (Nov 2).

In a 53-page report detailing atrocities in Mosul that amount to "international crimes", the United Nations said the executed civilians were among at least 2,521 who were killed during the battle for the city, mostly from ISIS attacks, including indiscriminate shelling and the use of improvised bombs and - increasingly - explosive-laden drones.

The report said the militants also carried out mass abductions of civilians, used thousands of civilians as shields in combat with Iraqi soldiers, and forcibly recruited boys as young as nine from families and then deployed them as "Cubs of the Caliphate" wearing explosive belts.

The report, produced by the UN Assistance Mission for Iraq and the Office of the High Commissioner for Human Rights, could be used for war-crimes prosecutions.

"During the course of the operation to retake Mosul city thousands of civilians were subjected to shocking human rights abuses and clear violations of international humanitarian law," Zeid Ra'ad al-Hussein, the high commissioner for human rights, said in releasing the report. "Those responsible must answer for their heinous crimes," he said.

Among the report's recommendations was allowing the International Criminal Court jurisdiction to pursue justice for victims. The report also said the Iraqi government could amend the country's criminal code to grant domestic courts jurisdiction over international crimes.

Seized by ISIS fighters in 2014, Mosul was the militant group's biggest urban conquest, with a population of 1.5 million, part of a span of territory it held in Iraq and neighboring Syria.

With air support from the US military and help from other allies, Iraqi forces began to retake the city in October 2016 and declared victory in July.

But much of Mosul was heavily damaged, including its famed al-Nuri mosque, and vast parts remain littered with unexploded bombs and booby-traps left by fleeing ISIS fighters.

The report was based on testimony of victims, survivors and witnesses to atrocities. While some implicated Iraqi forces in extrajudicial killings or abuse, the report attributed the vast majority of violations to ISIS.

A section of the report on mass killings and summary executions by the ISIS described three instances in which civilians accused of collaborating with the government were executed.

On Nov 25, 2016, the report said, militants shot to death 27 civilians in Muhandiseen Park for leaking information. On Dec 2, militants killed a Mosul University student for having received cash from a friend outside the city. On Dec 17, they publicly beheaded two civilians in eastern Mosul suspected of having cooperated with Iraqi forces.

Another section of the report, on the ISIS' use of improvised explosives, included attacks with car bombs and weaponised drones.

On Nov 6, 2016, it said, an explosive-laden vehicle in eastern Mosul killed 35 civilians, including women and children. On Feb 3, a drone carrying explosives hit an eastern Mosul primary school, killing two women teachers. A week later, an explosive-laden drone targeted a funeral in northern Mosul, killing eight, including three women.

The report said ISIS' use of weaponised drones grew more frequent in the course of the battle for Mosul and "represented a new tactic".