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.
Yazidis, whose religion includes elements of several faiths, are considered infidels by IS. (+ BBC video) » | Saturday, May 02, 2015
