Britain's spy chief voiced doubt on Thursday that the 2015 nuclear deal between Iran and world powers can be revived.

Speaking at the Aspen Security Forum in Colorado and quoted by AFP, MI6 chief Richard Moore said Iran’s Supreme Leader Ayatollah Ali Khamenei remains opposed to the deal, despite marathon diplomacy with the United States.

"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," said Moore.

"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," Moore said at the conference, in what was billed as his first public speaking appearance abroad.

"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," he continued, according to AFP.

Moore also alluded to Western intelligence's operations in Iran as he criticized Tehran's support for militant movements in Iraq, Syria and Yemen.

"Even if we do the deal, by the way, there's still plenty of work for my service to do because of what they're up to in terms of destabilizing activity around their region," he said.

Iran scaled back its compliance with the 2015 deal, in response to former US President Donald Trump’s withdrawal from the agreement in May of 2018, but has held several rounds of indirect talks with the US on a return to the agreement.

An agreement was nearly reached before the talks stopped in March. US Special Envoy for Iran Rob Malley told lawmakers recently that the prospects for reaching a deal with Iran are “tenuous” at best.

France's Foreign Minister, Catherine Colonna, said last week there were only a few weeks left to revive the deal and it was up to Iran to decide whether to sign what had been negotiated.

"The window of opportunity will close in a few weeks. There will not be a better accord to the one which is on the table," she warned.

Later, when asked if the United States concurred with Colonna's view, a senior US official pointed to Monday's comment by White House national security adviser Jake Sullivan to reporters that "we have not marked a date on the calendar."