Iran has conducted air strikes against Islamic State (IS) targets in eastern Iraq during recent days, a Pentagon spokesman says.
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.
Representatives of the US-led international coalition against IS are gathering at Nato headquarters in Brussels for a summit.
The talks chaired by US Secretary of State John Kerry will discuss military strategy as well as ways to stem the flow of foreign fighters joining IS and how to counter its propaganda, officials told AFP news agency.
The ministers meeting will discuss ways to send “counter-messages” to de-legitimise IS, which has been adept at using social media to promote its cause, an unnamed senior US state department official told AFP.
“As we are able to de-legitimise [IS], we are able to turn populations against [IS], reduce their recruiting basis,” he added.
The US has said it would be inappropriate for Iran to join the coalition, even though the two long-time adversaries face a common enemy in IS.
A jet filmed over Iraq by Qatari-based broadcaster al-Jazeera has been identified by Jane’s Defence Weekly as an Iranian Phantom.
Rear Adm Kirby’s comments followed reports that American-made F4 Phantom jets from the Iranian air force had been targeting IS positions in the eastern Iraqi province of Diyala.
“We have indications that they did indeed fly air strikes with F-4 Phantoms in the past several days,” he said.
It was up to Iraq to oversee and co-ordinate flights by different countries in its airspace, he added.
“We are flying missions over Iraq, we co-ordinate with the Iraqi government as we conduct those,” he said. “It’s up to the Iraqi government to deconflict that airspace.”
“Nothing has changed about our policy of not co-ordinating military activity with the Iranians.”
‘No place for US’
The Deputy Chief of Staff of Iran’s Armed Forces Brig-Gen Massoud Jazayeri also denied collaboration.
He said Iran considered the US responsible for Iraq’s “unrest and problems”, adding that the US would “definitely not have a place in the future of that country”.
Shia Muslim-ruled Iran has close ties to Iraq’s Shia-led government, which struggled to counter IS militants as they seized swathes of territory in eastern Syria and northern and western Iraq.
Military analysts said earlier this year that Iran was supplying Iraq with Soviet-built Sukhoi Su-25 attack jets to help in the fight against IS.
Shia militias trained and funded by Iran have also been sent to Iraq to support Kurdish fighters battling IS militants.
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. They were freed in 1981.