Story highlights A dozen people reportedly killed in ISIS captivity, UN refugee agency says

Group says ISIS uses captives as human shields in fight against Iraqi troops

(CNN) ISIS militants have captured up to 3,000 people displaced from their homes trying to flee conflict zones in northern Iraq, the United Nations said Friday, reportedly killing 12 of them.

More than 4 million people have been displaced within Iraq.

The UN refugee agency said it had received reports that ISIS rounded up the thousands of internally displaced people Thursday from villages as they tried to escape to the city of Kirkuk.

"Reportedly, 12 of the IDPs have been killed in captivity," the agency said in its daily Iraq update, using an acronym for internally displaced persons.

The battle for Mosul

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