The report 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 Daesh militants.
The observatory said Daesh was using people as shields against attacks by Iraqi security forces, adding scores of civilians had been executed, and six burnt.
The report said the Takfiri group had captured the 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.
The Iraqi army and allied volunteer forces are preparing for a final push to dislodge Daesh from Mosul - the Takfiri's group's last major city in Iraq.
The UN has appealed for about $2 billion of donations to deal with the aftermath of the Mosul assault but has received nothing so far.
The likely capture of thousands of Iraqis came as Izadis marked this week the second anniversary of what investigators have termed a genocide by Daesh against the Iraqi minority group.
On Wednesday, the United Nations said thousands of Izadis are being held captive by Daesh in Syria where many are used for sexual slavery or forced to fight for the Takfiri group.
According to the UN, around 3,200 Yazidi women and girls are being held captive, and thousands of men and boys are missing.
UN chief Ban Ki-moon on Wednesday condemned "the heinous crimes that continue to be committed by ISIL (Daesh) against the diverse ethnic and religious communities of Iraq."
He also expressed deep concern "about the safety of the people who remain in ISIL’s captivity," calling upon all those engaged in the fight against Daesh to make their release a prime objective in their military operations.
The US has been deploying additional troops to Iraq in anticipation of the assault on Mosul.
The plan has raised questions, with some critics citing Washington's failure to react when Daesh was overrunning Iraqi cities one after another.
The Americans and their allies have also been assisting Iraqi Kurds which maintain a self-rule on some portions of the Arab country.
Kurdish forces are among the groups primed for a push on Mosul along with the Americans.
Hezbollah deputy head Sheikh Naim Qassem warned this week against the partitioning of Iraq and neighboring Syria.
"On the battlefield and in view of regional and international interventions I don't rule out that one of the ideas proposed is finding a state of partition in those two countries but will it succeed or not?" he said in an interview with Reuters.
"So far the forces that want the unity of Iraq and Syria are able to prevent the idea of partition but we should remain worried about the possibility that some countries might push these two countries or one of them into partition;" Press TV reported.
218-11