Meanwhile Amnesty International says that it has uncovered new evidence that Islamic State have launched a campaign of ethnic cleansing in the north.
IS and allied Sunni rebels have seized large swathes of Iraq and Syria.
Thousands of people have been killed, the majority of them civilians, and more than a million have been forced to flee their homes in recent months.
"The Islamic State (IS) is carrying out despicable crimes and has transformed rural areas of [the northern region of] Sinjar into blood-soaked killing fields in its brutal campaign to obliterate all trace of non-Arabs and non-Sunni Muslims," Amnesty's Donatella Rovera said.
Amnesty says it has gathered proof that several mass killings took place in the northern region of Sinjar in August. Two of the deadliest took place when IS fighters raided villages and killed hundreds on 3 August and 15 August.
"Groups of men and boys including children as young as 12 from both villages were seized by IS militants, taken away and shot," the UK-based group said.
Deputy Human Rights Commissioner Flavia Pansieri warned that IS (formerly known as Isis) was targeting Christian, Yazidi, Turkmen, Shabak, Kaka'i, Sabean and Shia communities "through particularly brutal persecution".