Tehran (AFP) - Iran said Monday it rejected a US request for its cooperation against the jihadist Islamic State as part of an international coalition whose true aim Tehran sees as regime change in Syria.

Seen from Tehran, which has helped both Damascus and Baghdad to confront IS advances, the coalition lacks credibility because some of its members had financed and armed the group as part of their campaign to bring down Syrian President Bashar al-Assad.

"Right from the start, the United States asked through its ambassador in Iraq whether we could cooperate against Daaesh," Iran's supreme leader Ali Khamenei said in a statement on his official website, using the Arabic acronym for IS.

"I said no, because they have dirty hands," said Khamenei, who has the final say on all matters of state in the Islamic Republic.

"Secretary of State (John Kerry) personally asked (Iranian counterpart) Mohammad Javad Zarif and he rejected the request," said Khamenei, who was leaving hospital after what doctors said was successful prostate surgery.

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

At the end of a Paris conference on coordinating the fight against IS, to which Iran and Syria were not invited, the United States said Monday it was opposed to military cooperation with Iran in Iraq but was open to further talks.

"We will not be coordinating with Iran," Kerry told reporters shortly after Khamenei's statement. "But as I said, we are open to have a conversation."

For Khamenei, "the Americans are lying when they say they refused to have Iran in the alliance because from the very start we declared our opposition to such a presence".

Washington had appealed for help from all regional states against the jihadists, who spearheaded a lightning offensive through the Sunni Arab heartland north and west of Baghdad in June and then unleashed a wave of atrocities against ethnic and religious minorities.

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But last week Kerry ruled out cooperation with Tehran, citing the mainly Shiite country's "engagement in Syria and elsewhere".

Tehran has been the main regional ally of the Damascus government throughout a three-and-a-half-year armed revolt against Assad.

It strongly criticised President Barack Obama's announcement last Wednesday that he had authorised US air strikes against IS targets in Syria without the consent of Damascus.

- Like 'Friends of Syria' -

Khamenei predicted the US-led coalition against IS would prove as ineffective as the so-called "Friends of Syria" international conferences held as a show of solidarity with anti-Assad rebels.

"The alliance against Syria... didn't manage to do anything, and it will be the same thing in Iraq," said Iran's leader.

In Paris, the world's top diplomats pledged to support Iraq in its fight against IS militants by "any means necessary", including "appropriate military assistance".

Representatives from around 30 countries and international organisations, including the United States, Russia and China, took part.

Iraq's Foreign Minister Ibrahim al-Jaafari voiced "regret" in Paris that Iran had not been invited to the conference.

"We insisted that Iran be present. However, it's not us that took the decision. We regret the absence of Iran at this conference," he told reporters.

"All countries are affected by the Daaesh problem and Iran is a neighbouring country that has several times given us its support," he said.

British Foreign Secretary Philip Hammond, meanwhile, called for Iran to cooperate with the coalition even if it did not form part of the alliance.

"It was always unlikely that Iran would become a fully fledged member of the coalition but I think we should continue to hope that Iran will align itself broadly with the direction that the coalition is going," Hammond told reporters.

He said he hoped Iran would be "cooperative with the plans that the coalition is putting in place, if not actively a part of the coalition".