UNHCR says it has received reports 12 of the people have been killed
Islamic State fighters have captured up to 3,000 Iraqi villagers trying to flee to the city of Kirkuk, according to the United Nations refugee agency.
The UNHCR says they were from Hawiga District and 12 were killed by the jihadist group.
It followed a statement on Thursday from the Iraqi Observatory for Human rights, which claimed 1,900 civilians had been captured by more than 100 IS fighters.
They were using the people as human shields against attacks by Iraqi Security Forces, the statement said, adding tens of civilians had been executed, and six burnt.
The UNHCR said it had "received reports that ISIL captured on 4 August up to 3,000 IDPs (internally displaced people) from villages in Hawiga District in Kirkuk Governorate, trying to flee to Kirkuk city.
"Reportedly, 12 of the IDPs have been killed in captivity."
Millions of people have been displaced since IS took control of large areas of Iraq and neighbouring Syria in 2014.
The United States has been leading a military coalition carrying out airstrikes against IS, and the group's grip on some towns has been broken.
But it still controls Mosul in Iraq and Raqqa in Syria.
The UN has appealed for more than €200m to prepare aid in the event of an assault on Mosul, as well as up to €1.7bn to deal with the aftermath.
So far it has received nothing, according to the UN Financial Tracking Service.
UNHCR has begun building a site near Mosul for 6,000 people, and is preparing another for 15,000, but that will help a fraction of those expected to need shelter.
US President Barack Obama says Islamic State continues to be a threat to the US despite losing significant ground in Iraq and Syria.
Tens of thousands who fled from the Iraqi city of Fallujah have still not returned since its recapture from Islamic State in June.
UNHCR said three volunteers who were helping to clear Fallujah of rubble and explosives died while clearing a house earlier this month.
"Although local authorities have suggested that returns to Fallujah could begin in September, the Ministry of Migration and Displacement has stated that it may take another three months before conditions are conducive for large scale returns," it said.
The Iraqi authorities have said 300,000 displaced people had returned to Ramadi district, said UNHCR.
Iraqi forces declared victory over the jihadist group in Ramadi in December, but later called a halt to returns after dozens of civilians were killed by mines.