The Islamic State of Iraq and the Levant (ISIL) has killed at least 200 people from the Albu Nimr tribe, which had taken up arms against the fighters, police officers cited by news agencies said.
Women and children were said to be among those executed over the past 10 days in western Iraq's Anbar province which has been largely overrun by ISIL.
Accounts varied as to the number and timings of the executions in Anbar, but all sources spoke of more than 200 people murdered in recent days.
Police Colonel Shaaban al-Obaidi told the AFP news agency that more than 200 people were killed, while Faleh al-Essawi, deputy head of Anbar provincial council, put the toll at 258.
Canada joins campaign
News of the killings came as Canada conducted air strikes on ISIL positions in Iraq for the first time on Sunday.
"Today's strike demonstrates our government's firm resolve to tackle the threat of terrorism and to stand with our allies against ISIL's atrocities against innocent women, children and men," Canadian Defence Minister Rob Nicholson said in a statement.
Canada joined the anti-ISIL coalition on Thursday and conducted two days of reconnaissance before sending two CF-18s to attack jihadist positions around the city of Fallujah.
Iraq is bracing for yet more violence in the coming days as hundreds of thousands of Shia prepare to travel to shrines in Karbala for a major annual pilgrimage.
ISIL, a Sunni group that has seized large parts of Iraq and Syria, is expected to target Ashura pilgrims; 19 people died in attacks on Shia Muslims on Sunday.
The killings are probably aimed at discouraging resistance from powerful local tribes in Anbar.
ISIL also detained dozens of members of the Jubur tribe in Salaheddin province, north of Baghdad, officials and a tribal leader said.
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Jubur tribesmen and security forces have been holding out for months against ISIL in the provincial town of Dhuluiyah.
Pro-government forces have suffered a string of setbacks in Anbar in recent weeks, prompting warnings that the province, which stretches from the borders with Jordan and Saudi Arabia to the western approach to Baghdad, could fall entirely.
Security forces who wilted before a lightning ISIL offensive in June are fighting to retake territory seized by the jihadists in Iraq's Sunni Arab heartland.
ISIL has declared a "caliphate" in territory it controls, imposing its harsh interpretation of Islamic law and committing widespread atrocities.
Like other armed Sunni groups, ISIL considers Shia Muslims to be heretics and frequently attacks them, posing a major threat to the Ashura religious commemorations which peak on Tuesday and will be a major test for the new government headed by Prime Minister Haider al-Abbadi.