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The failure of efforts to restore the pact could carry the risk of a regional war, or lead to more harsh Western sanctions on Iran and continued upward pressure on world oil prices that are already high due to the Ukraine conflict, analysts say.
"I can't be confident it is imminent.. a few months ago we thought we were pretty close as well," Malley said at the Doha Forum international conference.
"In any negotiations, when there's issues that remain open for so long, it tells you something about how hard it is to bridge the gap."
His assessment of the negotiations in Vienna to revive a 2015 nuclear accord came after Kamal Kharrazi, a senior advisor to Iran's Supreme Leader Ayatollah Ali Khamenei, said a deal could come soon.
Asked about any potential redesignation, Malley said: "Regardless of what happens to the IRGC issue that you raise, our view of the IRGC is many other sanctions on the IRGC will remain. This is not a deal that intends to resolve that issue."
Tehran has also been pushing for guarantees that any future U.S. president would not withdraw from the deal, which would curb Tehran's nuclear programme in exchange for lifting tough sanctions which have hammered Iran's economy.
The extent to which sanctions would be rolled back is another sensitive subject.
Enrique Mora, the EU coordinator for the nuclear talks, had said on Friday he would travel to Tehran on Saturday to meet Iran's chief nuclear negotiator.
