US Secretary of State John Kerry on Monday ruled out military cooperation with Iran but said Washington remained open to talks with Tehran about the threat posed by Islamic State militants.

"We will not be coordinating with Iran" on military action, "but as I said, we are open to have a conversation," Kerry told reporters in Paris after an international conference on Iraq.

"On Syria strikes, we are not going to coordinate with the Syrians, we have made that very, very clear, but there are all kinds of ways of communicating to avoid mistakes or disaster," he added.

US President Barack Obama has outlined a strategy to combat the Islamic State extremists, including air strikes in Syria and expanded operations in Iraq, where US aircraft have carried out more than 160 strikes since early August.

But Washington had steadfastly ruled out tipping off the regime of President Bashar al-Assad about any potential strikes in Syria, where militants hold a quarter of the land.

State Department spokeswoman Jen Psaki had told reporters earlier that there "may be another opportunity on the margins in the future to discuss Iraq" with the Tehran authorities.

Iran was not invited to the Paris conference and later said it had no wish to attend the gathering that it said was purely "for show".

Iran's supreme leader Ali Khamenei then revealed he had rejected a US request for cooperation in the fight against the militants because the Americans had "dirty hands".

He accused Washington of seeking a "pretext to do in Iraq and Syria what it already does in Pakistan -- bomb anywhere without authorisation."

Western diplomats have voiced concern privately that Iran wants to link its cooperation in the fight against jihadists to negotiations over its disputed nuclear programme.

On the nuclear talks, Kerry said: "I am hopeful it will possible to find a way to reach an agreement that is important to the world. But there are very difficult issues" still to be resolved.