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 Daesh," Iran's supreme leader Ali Khamenei said in a statement on his official website, using the Arabic acronym for IS.
"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."
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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 today it was opposed to military cooperation with Iran in Iraq but was open to further talks.
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.