A Pentagon investigation has found that Iran-backed Shiite militias got their hands on US-made M1A1 Abrams tanks and used them against US-backed Kurdish forces last fall.

The US military newspaper Stars and Stripes says a recently released quarterly inspector general report on the anti-Islamic State mission cited reports that the pro-Iran militias, known as Popular Mobilization Forces, or PMF, had obtained up to nine of the US main battle tanks which they used in combat.

The PMF, which were fighting as part of the Iraqi security forces, had appropriated the tanks from ISIS in Tikrit in 2015 and in Mosul last year. ISIS had originally captured the armored vehicles from the Iraqi Army when it made its initial advances in Iraq.

US legislators reportedly threatened to curb military aid to Iraq when they learned that US tanks had been used against the Kurds, who are close US allies.

The Iraqi government has regained control of at least some of the tanks since October, the Pentagon report said. Other media reports were cited as saying they were all returned.