Isis is feared to have captured up to 3,000 civilians fleeing the group’s territory in Iraq.
The UN Refugee Agency said militants had killed at least 12 people, who had been moving from villages in al-Hawija district to seek safety in Kirkuk city on Thursday.
A security source told the Iraqi News website they died when Isis fighters “fired openly” on families as they tried to flee.
“Three thousand civilians, including women, men, children and elders tried to escape this afternoon from Hawija,” he added.
“Isis members ambushed the civilians and prevented them from escaping.”
They were believed to be trying to escape Isis-controlled territory by moving towards Kurdish Peshmerga forces who are attempting to drive militants out of the region.
Hawija lies between the provinces of Kirkuk, Nineveh and Salahaddin has an estimated 400,000 inhabitants, many of whom have been displaced by the war.
The district has been one of the hotspots for the insurgency that started in 2004.
Much of al-Hawija district is currently under Isis control, with the Peshmerga attempting to move in from Kirkuk to the east and Iraqi forces holding a frontline around Baiji to the west.
Britain and other members of the US-led international coalition have been conducting regular strikes on Isis territory around Kirkuk and in other areas of northern Iraq.
The terrorist group has suffered a series of major losses in the country, including the city of Fallujah, and operations are expected to turn to its de-facto capital of Mosul.