Iran on Thursday denied linking any future cooperation with the international community against jihadists in Iraq to the lifting of crippling Western sanctions.

Earlier, the official IRNA news agency quoted Foreign Minister Mohammad Javad Zarif as suggesting that Tehran could help the international community if it lifted the sanctions.

"If we agree to do something in Iraq, the other side of the negotiations should do something in return," IRNA had quoted Zarif as saying.

"All the sanctions that are related to Iran's nuclear programme should be lifted."

His reported comments followed a call by French Foreign Minister Laurent Fabius on Wednesday for all countries in the region, including Iran, to join the fight against Islamic State (IS) fighters who have seized swathes of Iraq as well as parts of neighbouring Syria.

But the foreign ministry said the words attributed to Zarif were "not correct".

"The news on the quote of foreign minister about Iran and US collaboration on Iraq for lifting sanctions are not correct," an English-language statement said.

"This issue (Iraq) was not brought up and the news on this are invalid," it said.

Zarif, it said, was talking about the Arak research reactor -- a source of concern for the international community because by-product waste from there could provide an alternative route to an atomic bomb.

Iraq and Arak can sound the same in Farsi.

Crippling EU and US sanctions are the subject of ongoing talks between Tehran and the major powers that are due to resume before the UN General Assembly opens next month.

In return for lifting the sanctions, the Western powers are demanding that Iran sharply rein in its nuclear programme to allay international concerns about its ambitions as part of a comprehensive deal they are seeking to strike by November.

Another Iranian news agency, Mehr, had quoted Zarif as saying that tough negotiations were still under way over what role Iran might play in Iraq and what the reward might be for its cooperation.

"It is still not clear what we have to do in Iraq and what they have to do in return," Mehr quoted the Iranian foreign minister as saying.

"And that's exactly the difficult part."

Iranian and US officials discussed the jihadists' lightning offensive in Iraq in June on the sidelines of nuclear talks with the major powers, but both sides ruled out joint military action at the time.

Tehran and Washington have had no diplomatic relations since the aftermath of the Islamic Revolution of 1979, although they have had contacts over Afghanistan as well as Iraq.