Amnesty International says it has gathered evidence that shows Islamic State jihadists have been carrying out what it calls a systematic campaign of ethnic cleansing in northern Iraq.
The report says the fighters are committing war crimes against ethnic and religious minorities.
The human rights group said IS had turned the region into "blood-soaked killing fields".
The BBC reports the UN earlier announced it was sending a team to Iraq to investigate "acts of inhumanity on an unimaginable scale".
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.
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.
"IS is carrying out despicable crimes and has transformed rural areas of Sinjar into blood-soaked killing fields in its brutal campaign to obliterate all trace of non-Arabs and non-Sunni Muslims."
On Monday, the United Nations Human Rights Council agreed to deploy an emergency mission to investigate crimes allegedly carried out by IS.
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".