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.
The operation to liberate Mosul has been months in the making and involves coordination between Iraq's military, Kurdish forces and the US-led airstrikes.
There are more than 4 million internally displaced persons in Iraq, according to data from the UN refugee agency, which said the fall of Ramadi in May 2015 "resulted in significant levels of new displacement, mostly in Anbar Governorate." Iraq subsequently recaptured Ramadi from ISIS.
UN Secretary-General Ban Ki-moon said Wednesday that ISIS had committed "heinous crimes" that may constitute war crimes and genocide.