BERLIN -- The U.N. refugee agency says it has received reports that the Islamic State of Iraq and Syria (ISIS) group may have captured up to 3,000 Iraqis who were fleeing violence and that 12 of them may have been killed.

In a report published Thursday, the U.N. High Commissioner for Refugees said the internally displaced people from villages in the Hawiga district had been trying to flee to the northern Iraqi city of Kirkuk when they were captured by ISIS.

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

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ISIS fighters have repeatedly prevented civilians from fleeing territory that the militant group holds, using them as human shields against airstrikes. Iraqi government ground forces still rely heavily on U.S.-led coalition airstrikes in the fight against ISIS. 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.