IS skal fredag ha drept 300 yazidier i Tel Afar, ifølge yazidi-kilder og irakiske regjeringskilder.
A statement from the Yazidi Progress Party said 300 captives were killed on Friday in the Tal Afar district near the city.
Iraqi Vice-President Osama al-Nujaifi described the reported deaths as «horrific and barbaric».
Thousands of members of the religious minority group were captured last year.
It is not clear how they were killed, or why this has happened now, says the BBC’s Middle East editor Alan Johnston.
Many are reported to have been held in Mosul, the main stronghold of IS after the militants swept through large areas of northern and western Iraq, and eastern Syria in 2014.
Yazidiene utgjør bare 500.000. Det er mye overtro om dem blant muslimer. Mange mener de er djevle-tilbedere og IS tar konsekvensen av dette og forsøker å utrydde dem.