A group of alleged Shiite militia fighters executed a young boy in the Sunni Iraqi city of Tikrit. They accused him of being a member of the Islamic State group, and released an amateur video of the killing on Facebook Tuesday. Because of the boy's youth, the video sent shock waves through Western media. But beheadings, bullet-riddled bodies and images of child soldiers are posted almost daily on social media sites affiliated with Iran-backed Shiite militias in Iraq.

Tikrit has been in a state of sectarian conflict for years and the rise of the group also known as ISIS and the Iran-backed Shiite militias now threatens to pull the rest of the country into sectarian chaos. On Monday, Iranian-backed Shiite militias joined the U.S.-backed Iraqi Armed Forces in one of the largest offensives to push back the ISIS militants from Tikrit, a largely Sunni city and Saddam Hussein’s hometown.

The Iran-backed Shiite “Popular Mobilization” forces have been fighting Sunnis -- not just ISIS -- in Sunni-dominated areas for months, claiming to seek revenge for ISIS attacks on Shiites and decades of persecution under Hussein's brutal Baathist regime. Since June, Shiite militias have reportedly abducted and executed hundreds of Sunni Iraqis, whom they accuse of being ISIS members.

“Shiite militias have been taking advantage of the atmosphere of lawlessness and impunity to abduct and kill Sunni men, seemingly in reprisal or revenge for IS [ISIS] attacks and at times also to extort money from the families of those they have abducted,” according to an Amnesty International report.

Many of the larger Shiite militias have their own social media pages and a website to publish brutal footage and photos of their operations A quick scan of their pages shows many of the same brutal acts that have been committed by ISIS.

“It depends on where you’re looking. I found a ton of beheadings; one rivals anything I’ve ever seen on the Sunni side. I’ve found them dragging bodies behind cars,” said Phillip Smyth, a University of Maryland researcher who studies ISIS and Shiite militias. “They have no real problem with putting this stuff up.”

When ISIS invaded the country this summer, it, too, fueled historic tensions between the Sunnis and the Shiites, hoping to gain Sunnis' allegiance.

“This business of Daesh (ISIS) and the war it has caused have poisoned relations among and within communities,” an Iraqi from Kirkuk whose son was reportedly abducted and killed by a Shiite militia told Amnesty International. "Sunni or Shi’a did not use to matter; now some people are exploiting the situation and causing dangerous divisions.”

There are dozens of active Iraqi Shiite militias under four separate umbrella organizations with some degree of relationship to Iran. At least six of Iraq’s Shiite militias have ties to Lebanon’s Iran-backed organization Hezbollah and at least nine have ties to the Badr Organization, an Iraqi political party whose military wing is commanded by Iran.

On the frontlines of Monday’s operation was Asaib Ahl al-Haq, the League of the Righteous, one of the strongest Shiite militias in Iraq today. The militia’s leader, Qays al-Khazali, was seen with the leader of Iran’s Quds Force, Gen. Qasem Soleimani, on the frontlines. The men were joined by the leader of Kataib Hezbollah, a group with ideological ties to Hezbollah in Lebanon that was added to the U.S. Foreign Terrorist Designation list in 2009.

“They can look and operate like regular armed forces but are not regular by any laws or subject to oversight and accountability mechanisms,” said Amnesty International, which has accused the militias of committing war crimes.

Larger brigades can have tens of thousands of fighters, mostly made up of Shiite Iraqi combatants who have been trained in Syria, Lebanon and even Iran. Their combined forces now range between 100,000 and 250,000, according to the Washington Post.

“The more the militias become successful and expand their areas of operations, the more their influence increases and the state’s decreases,” according to a recent report on Shiite militias from the Institute for the Study of War (ISW). “Should military operations render ISIS no longer an existential threat, these militias would greatly threaten the sovereignty of Iraq.”

Correction: An earlier version of this story incorrectly claimed that Tikrit was in Anbar province. It is located in Salahuddin province.