“We have indications that they did indeed fly air strikes with F-4 Phantoms in the past several days,” Pentagon spokesman Rear Admiral John Kirby told AFP.
His comments came after Al-Jazeera recently ran footage of what appeared to be an F-4 fighter, similar to those used by the Iranian air force, attacking targets in the eastern province of Diyala.
At a press conference earlier, Kirby said it was up to the Iraqi government to oversee and coordinate military flights by different countries and not US commanders.
“We are flying missions over Iraq. We coordinate with the Iraqi government as we conduct those. It’s up to the Iraqi government to deconflict that air space,” Kirby told reporters.
“Nothing has changed about our policy of not coordinating military activity with the Iranians.”
Iranian forces have reportedly been active on the ground in Iraq assisting Shiite militia and Baghdad government units, but this was the first time the United States had confirmed the Iranians were conducting air strikes against the IS group.
Iran also has provided Sukhoi Su-25 aircraft to Iraq amid widespread speculation that the planes are flown by Iranian pilots.
Confirmation of the Iranian air raids offers the latest illustration of how the threat posed by the IS extremists has brought the two arch-foes together in the fight against the jihadists.
Shiite-ruled Iran has close ties to the Shiite-led government in Baghdad, which has struggled to counter the IS group.
US fighters, bombers and surveillance aircraft fly daily missions over Iraq along with other coalition warplanes from European governments as well as Australia and Canada.
The US-led air war against the IS group began on August 8 in Iraq and was extended into Syria in September.