"If we can get a deal it's probably the best means still available to constrain the Iranian nuclear program. I'm not convinced we're going to get there," MI6 chief Richard Moore told the Aspen Security Forum.

"I don't think the supreme leader of Iran wants to cut a deal. The Iranians won't want to end the talks either so they could run on for a bit," he said at the conference in the US state of Colorado, in what was billed as his first public speaking appearance abroad.

President Joe Biden has backed reviving the 2015 agreement which was negotiated under former US leader Barack Obama and trashed three years later by Donald Trump.

But indirect talks in Vienna have dragged on, in part over the US rejection of Iranian demands to reverse Trump's blacklisting of the powerful Revolutionary Guards as a terrorist group.

"I think the deal is absolutely on the table. And the European powers and the administration here are very, very clear on that. And I don't think that the Chinese and Russians on this issue would block it. But I don't think the Iranians want it," Moore said.

Britain, China, France, Germany and Russia were all parties to the deal, formally known as the Joint Comprehensive Plan of Action, under which Iran drastically scaled back nuclear work in exchange for promises of sanctions relief.

Khamenei, who has led the Shiite clerical state since 1989, is known for his hostility toward the United States, a central tenet of the Islamic Revolution a decade earlier that toppled the Western-oriented shah.

He had reluctantly backed negotiators under the reformist government of president Hassan Rouhani who reached the deal in 2015, but Trump's withdrawal and reimposition of sweeping sanctions on Iran were seen by hardliners as confirmation of US ill intent.