The Islamic State has liquidated 300 members of the Yazidi religious minority west of Mosul, according to Yazidi and Iraqi officials, as reported by the BBC.
The Yazidi Progressive Party said in a statement that 300 prisoners were killed on Friday in the Tal Afar district near the city.
Iraqi Vice President Osama Al Nujaifi described the allegations of the killings as "horrific and barbaric".
Several thousand members of the Yazidis were captured last year.
It is not clear how they were liquidated or why it happened now, says BBC Middle East editor Alan Johnston.
According to some reports, the Islamic State is holding most of the captives in Mosul, its main stronghold.
The Yazidis, a minority whose religion incorporates elements of several different religions, are also considered sinners by the Islamic State.
Several hundred men have been killed so far, while some women have been kept alive to be sex slaves, according to the BBC.
The Yazidi Progressive Party condemned the "horrible crime" and called on Iraqi forces to free those still held captive by the Islamic State, according to local media.
In January, the Islamic State released about 200 mostly elderly Yazidis.
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