Marking another twist in a faltering offensive on Tikrit, key Shiite militias involved in the operation expressed anger that the U.S. is launching air strikes to back the operation, and according to reports, thousands of combatants are boycotting the fight on those grounds, with some threatening to attack U.S. forces.

McClatchy reported

Launched over three weeks ago, the operation on the hometown of Saddam Hussein is the largest counter-offensive against ISIS yet, involving tens of thousands of Iraqi and Shiite militia fighters. This force is dominated by Shiite militias, which outnumber Iraqi troops 6 to 1 around Tikrit, according to the Washington Post.

The U.S. initially did not take part in the attack, with Iranian advisers playing a much more visible role, including Qasem Soleimani, head of the Iranian Revolutionary Guard’s overseas unit.

As the offensive stalled, the U.S. began launching air strikes to back the operation late Wednesday night.

Pentagon officials have sought to publicly downplay—and even deny—de-facto U.S. collaboration with Shiite militias and Iran.

SCROLL TO CONTINUE WITH CONTENT