The US has been leaning heavily on militias in its fight against ISIS in Iraq and Syria, and while these forces have proved very effective on the ground, some have been accused of committing atrocities akin to their enemies.

A new Human Rights Watch report details allegations of torture and abuse at the hands of Shia militias in Iraq, which have been instrumental in aiding Iraqi Security Forces in seizing territory back from ISIS (also known as the Islamic State, ISIL, or Daesh).

The organization has "received credible allegations of summary executions, beatings of unarmed men, enforced disappearances, and mutilation of corpses by government forces" that have been fighting on the outskirts of Fallujah to retake the city from ISIS, according to the report.

Here are some of the allegations from the report:

Human Rights Watch interviewed witnesses to one alleged atrocity near Fallujah who said Shia militia fighters and federal police had "separated men from women, marched the men to where the troops' officers were, lined them up, and shot at least 17 of them, including one teenage boy."

Another witness told the organization that he saw Shia militia fighters and federal police near Fallujah "fatally shoot civilians with white flags raised fleeing toward the government forces." He claims one fighter "told him his superior officer had ordered the shootings."

Some men whom Shia militias had recently released from detention "showed signs of torture, including rape, burns, knife cuts, and bruising from beatings."

The abuse allegations, which have become widespread in Iraq, are deepening the very sectarian tensions that facilitated the rise of ISIS in the first place. ISIS is a Sunni terror group that markets itself as a protector of Sunni civilians who have become targets of Shia militias, which have the support of the Shia-dominated Iraqi government.

Iran, a Shia theocracy and ally of Iraq, also backs these militias as it attempts to extend its influence in the Middle East.

As the Human Rights Watch report makes clear, civilians often end up caught in the middle of this sectarian power struggle.

The Iraqi government, wanting to keep Iran happy, hasn't done much to rein in these militias. As a result, they often go unchecked, using their power to suppress Sunnis.

"It's a concerted effort [on behalf of the Shia militias] to punish the Sunni population for, in their minds, accepting ISIS or allowing ISIS," Michael Pregent, an adjunct fellow at the Hudson Institute and former US Army intelligence officer in Iraq, told Business Insider.

"But in reality [Sunnis] had nowhere to go. They couldn't call the police. They couldn't call the army."

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And it doesn't help that Fallujah has historically been a stronghold for extremists in Iraq.

Phillip Smyth, a researcher at the University of Maryland who is a leading expert on Shia militias, told Business Insider that people in Fallujah "gave Americans problems" during the Iraq War, and "gave the Iraqi government problems," so the militias' solution to routing the extremists "is to just smash" everything.

The Human Rights Watch report notes a video posted online that shows a Shia commander telling a room full of fighters that "Fallujah had been a bastion of terrorism since 2004 and that no civilians or true Muslims were left inside the city."

Treating all civilians in Fallujah as ISIS sympathizers plays right into ISIS' hand. If civilians are targeted by the militias that are supposedly liberating them from terrorist control, they become even more likely to turn against the Iraqi government.

"If the US continues to support Iraqi units that integrate Shia militias into their operations, they're simply resetting the conditions that led to ISIS to begin with," Pregent said. "You're further entrenching Sunni distrust of their government … and the United States."

These Shia militias are no friend to the US, either.

"These are not allies," Pregent said. "Everybody keeps thinking that these militias are working on a friendly basis with the US. They still hate us as much as they hate ISIS."

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