A report by campaign group Amnesty International has found that at least 170 Sunni Muslims have been abducted and killed by Shia militias around Baghdad as revenge against ISIS.
The Islamic State, which is a Sunni militant group, has beheaded many Shia Muslims, besides targeting Christians and Yazidis in Iraq.
The 24- page Amnesty International report also has accused the Iraqi government of supporting and even aiding the militias groups.
Earlier last month, even the new Iraqi Prime Minister Haider al-Abadi had admitted to previous "excesses" by security forces.
The report claimed that it has the details of more than 170 Sunni men who have been abducted from areas around Samarra, a city with a majority Sunni population north of Baghdad.
In the month of June, the report stated that more than 30 men were kidnapped from near their homes and were mercilessly shot dead. The incident reportedly was in response to ISIS fighters entering the city the previous day.
An unnamed Iraqi government official told Amnesty: "Sunni men who come from, go to, or live near areas where there are [ISIS] groups, tend to be considered by many militias to be terrorists or terrorist supporters and that is why they often get killed, whereas some militia-men target Sunnis in blind revenge for the crimes committed by Sunni terrorist groups."
The report noted that Shia militia groups, including Asa'ib Ahl al-Haq, the Badr Brigades, the Mahdi Army, and Kata'ib Hizbullah, have been accused of atrocities against Sunnis.
The militia groups that have thousands of enlisted Iraqi men wear military uniforms "but operate outside any legal framework and without any official oversight", Amnesty added.
"Shia militias are ruthlessly targeting Sunni civilians on a sectarian basis under the guise of fighting terrorism," said Amnesty International's senior crisis response adviser Donatella Rovera.
"The new Iraqi government of Prime Minister Haider al-Abadi must act now to rein in the militias and establish the rule of law."