But a spokesman said the US had not coordinated the attacks or military action with Tehran reported SKY News.
Rear Admiral John Kirby said the US continued to fly missions over Iraq and that it was up to the Iraqi government to avoid conflicts in its own airspace.
He said it was believed to be the first time Iran has launched manned aircraft from inside Iran to strike targets in Iraq.
Iranian military leaders have admitted that dozens of their forces have been in Iraq fighting alongside Kurdish troops battling extremists.
The US has not invited Iran to join the coalition fighting Islamic State and Iran has said it would not join in any case.
Rear Admiral Kirby said: “There’s no reason to believe they are not true, those reports that Iranian aircraft struck targets against IS in eastern Iraq.
“It’s Iraqi airspace and it’s for Iraqis to deconflict.
“We are not coordinating with nor deconflicting with Iranian military.”
It came as NATO foreign ministers met in Brussels to discuss the threat posed by IS.