This story originally appeared at 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.

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 Iraqi Observatory for Human Rights said Thursday that an estimated 100 to 120 ISIS militants had captured around 1,900 civilians.

It said the militants were using the captives as human shields in fighting against Iraqi forces.

The move is the latest by ISIS militants to retain a grip on swaths of the country as Iraqi forces, backed by US-led airstrikes, try to recapture Mosul, ISIS’ most significant stronghold outside Syria.