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GENEVA — ISIS fighters may have captured up to 3,000 fleeing Iraqi villagers on Thursday and subsequently executed 12 of them, according to the U.N. refugee agency.

The the U.N. High Commissioner for Refugee said it had received reports of the alleged incident.

It followed a statement on Thursday from the Iraqi Observatory for Human Rights, which said about 1,900 civilians had been captured by an estimated 100-120 ISIS fighters. They were said to be using people as shields against attacks by Iraqi security forces.

Women fleeing ISIS violence arrive on the outskirts of Al-Shirqat, Iraq, on July 30. Reuters

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"UNHCR has received reports that [ISIS] captured on 4 August up to 3,000 IDPs (internally displaced people) from villages in Hawija District in Kirkuk Governorate trying to flee to Kirkuk city. Reportedly, 12 of the IDPs have been killed in captivity," the UNHCR report said.

UNHCR officials in Geneva and Baghdad told The Associated Press on Friday they were still trying to verify the information and wouldn't comment on the source of the reports.

Related: New 'Heat Map' Shows ISIS Branches Spreading Worldwide

When ISIS first overran much of north and western Iraq in 2014, the extremists took thousands of women and children hostage for use as slaves or child soldiers.

Despite a string of defeats in Iraq and Syria, ISIS is still estimated to hold thousands of women and children captive, according to U.N. report earlier this week.

The U.S. is leading a military coalition conducting airstrikes against ISIS in Iraq and Syria, where the group seized broad swathes of territory in 2014. The fighting had displaced 3.4 million people in Iraq by July 2016.

ISIS' grip on some towns has been broken, but it still controls its de facto capitals of Mosul in Iraq and Raqqa in Syria.

Last month the U.N. appealed for $284 million to prepare aid for an assault on Mosul, as well as up to $1.8 billion to deal with the aftermath.

It has so far received nothing in response, according to the U.N. Financial Tracking Service.

UNHCR has begun building a site northeast of Mosul for 6,000 people and is preparing another northwest of the city for 15,000, a fraction of those expected to need shelter.