Iran has conducted air strikes against Islamic State (IS) targets in eastern Iraq during recent days, a Pentagon spokesman says, BBC reported.
Rear Adm John Kirby said the US, which has conducted its own air strikes in Iraq, was not co-ordinating with Iran.
A senior Iranian military official also dismissed talk of co-operation between the two countries.
A US-led coalition has launched hundreds of air strikes against IS since August.
The US has said it would be inappropriate for Iran to join that coalition, even though the two long-time adversaries face a common enemy in IS.
Since the Islamic revolution in 1979, the US and Iran have had a fraught relationship.
Washington severed ties the following year after Iranian students occupied the US embassy in Tehran and took 52 Americans hostage.