The militants "executed 36 people, including four women and three children" on Sunday in Anbar province, west of Baghdad, Albu Nimr tribal leader Sheikh Naim al-Kuoud al-Nimrawi told AFP.
Police Colonel Shaaban al-Obaidi put the number of dead at 50.
Both he and Nimrawi said that hundreds of members of the tribe were unaccounted for, with the colonel saying he had received reports that IS had kidnapped 1,000 of them and was executing 50 each day.
"We have more than 1,000 people about whom we do not know a thing now," he said.
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The tribal chief, however, said the campaign of killings was ongoing. "They issued a fatwa (religious ruling) for executing even infants from the Albu Nimr tribe," he said.
Accounts differ as to how many members of the tribe have been killed, with figures ranging from more than 250 to over 400 including the latest executions.
IS has overrun large areas of Anbar, and the killings are likely aimed at discouraging resistance from powerful Sunni local tribes, who will be key to any successful bid to retake the province.