Accounts varied as to the exact period in which the Sunni Albu Nimr tribesmen were killed in various areas of Iraq's Anbar province, but the executions were all said to have been carried out within the last 10 days.
Police Colonel Shaaban al-Obaidi told AFP that there were more than 200 people killed, while Faleh al-Essawi, the deputy head of the Anbar provincial council, put the toll at 258.
The victims, "including women and children, all of them from the Albu Nimr tribe," were killed "during the past three days," Essawi said.
Sheikh Naim al-Kuoud al-Nimrawi, a leader of the tribe, said that 381 of its members were killed "from the 24th of last month until today."
IS has overrun large areas of Anbar, and the killings are likely aimed at discouraging resistance from powerful local tribes, who will be key to any successful bid to retake the province.
Pro-government forces have suffered a string of setbacks in Anbar in recent weeks. That has prompted warnings that the province, which stretches from the borders with Jordan and Saudi Arabia to the western approach to Baghdad, could fall entirely.
One picture that has been circulating in the last week shows a line of more than 30 men in civilian clothes lying in the middle of a street with streams of blood running over the dusty ground, as young men and children look on.
The victims are barefoot and many are blindfolded, their hands bound behind their backs.
IS did not immediately claim responsibility for the killings, but has executed hundreds of people in areas of Iraq and Syria that it controls.
IS also executed hundreds of members of a Syrian tribe that fought against them, according to the Syrian Observatory for Human Rights monitoring group.