The Islamic State captured up to 3,000 internally displaced Iraqis who were fleeing from a village near the city of Mosul which is still under ISIS control.
The militants executed about 12 people, and burned six others, according to Iraqi Observatory for Human Rights.
An earlier report by the observatory said that around 1,900 people were seized by 110-120 ISIS fighters while fleeing Hawija district in Kirkuk province. The UN refugee agency received a report that the number of fleeing people is up to 3,000 but the officials have yet to verify the account.
The kidnapped civilians included women and children as well, who were attempting to escape on foot to safer areas on August 4.
An eyewitness told the Iraqi Observatory that "a number of women and children died [in Hawija] because of the lack of food, medicine, water, and electricity."
The agency asked the Iraqi government and the United Nations "to end the crisis in the area and to help rid its residents from the oppression of ISIS while ensuring the delivery of humanitarian assistance to residents."
"Many of the IDPs die and get wounded by stepping on Da'esh [ISIS] bombs [improvised explosive devices] while escaping from Hawija," a source told Kurdistan24.
Deputy governor of Kirkuk province, Rakan Saeed, asked Prime Minister Haider al-Abadi to rescue the captured civilians, but the government did not comment on the crisis.
The UN refugee agency is building two camps in northeast and northwest of Mosul, anticipating the arrival of thousands of people escaping the city.
ISIS captured large parts of Iraq and Syria in 2014, and kidnapped thousands of women and children to be used as slaves.
Many towns and cities have been taken back from the self-proclaimed caliphate (which is also sometimes called Daesh), but it still retains Mosul in Iraq and Raqqa in Syria. The IS has used civilians attempting to flee their territory as human shields against battle with opponents in the past.
The UN estimates say that the struggle to take back seized lands from the militant organization has displaced over 3.4 million Iraqis. The battle to retake Mosul could displace about 1 million more civilians, according to Red Cross.