As many as nine US tanks provided to Iraq’s military for the fight against the Islamic State (IS) have ended up in the hands of Iranian-backed militants, a government audit revealed on Monday.

The latest quarterly inspector general report for the US mission in Iraq and Syria confirms a string of on-the-ground reports that M1 Abrams battle tanks and other lethal equipment provided by the US government have ended up with the Popular Mobilization Units (PMU). The news adds credence to recent reporting by Iraq’s Al-Ghad news agency that Abrams manufacturer General Dynamics has suspended maintenance support for 160 of its tanks amid allegations that the Iraqi Security Forces (ISF) broke an agreement on their use.

The challenge of keeping US-supplied weapons and equipment out of the hands of groups with ties to Iran has grown over the course of the three-year mission, the report said, even as the State Department has continued to push Prime Minister Haider al-Abadi’s government to keep US arms confined to ISF units. In December, Abadi announced plans to cut back the Iranian-backed PMU, taking away some arms and vehicles.

This isn’t the first time that US weapons, including vehicles, have found their way to Iran-backed groups. In 2015, Iraqi Hezbollah brigades showed up in YouTube videos driving American-made M1 Abrams tanks, armored personnel carriers, Humvees and Canadian MRAP all-terrain vehicles, sparking concerns from Senate Armed Services Committee Chairman John McCain, R-Ariz.