A video released by the Islamic State group (ISIS) shows four prisoners in orange jumpsuits, suspended by their hands and feet, being burned alive. The men – members of a Shiite pro-government militia in Iraq – were killed in retaliation for the deaths of at least four ISIS members earlier this month.

"Now retribution has come, for today, we will attack them as they attacked us and punish them as they punished us," a masked ISIS militant said in the video, the Agence France-Presse reported Monday. The video does not disclose the time or location of the killings.

The video contained a clip from a video released last week by a Shiite militia showing a captured ISIS militant burned to death and then sliced up by a rebel fighter nicknamed Abu Azrael (“Father of the Angel of Death”), a poster boy of Iraqi pro-government brigades, Mirror UK reported. The four men were made to watch the video before being burned alive.

Abu Azrael, whose real name is Ayyub al-Rubaie, leads the Imam Ali Brigades, part of the pro-government Popular Mobilization forces. Fighters with the Shiite brigades have been accused of various human rights violations, particularly in areas recaptured from ISIS, by human rights leaders.

The Iraqi government and its allied Shiite militias have launched an aggressive campaign against ISIS in recent months. The Islamic State group conquered significant territory in Iraq last year and continues to control much of Iraq, including its second largest city, Mosul. ISIS forces have set up their own police forces and court systems in the areas under their control and adhere to a narrow and puritanical interpretation of Islam.

ISIS has been accused of egregious rights violations and atrocities across much of the territory under its thumb in Iraq and Syria, including mass executions of opponents and the enslavement of girls and women. Members of the group have filmed many of their atrocities – including beheadings and burnings – and posted them online as propaganda.