Richard Moore, chief of the Secret Intelligence Service (SIS) known as MI6, said he still believed that reviving the 2015 Joint Comprehensive Plan of Action (JCPOA) agreement was the best way to constrain Iran's nuclear program.

Under the deal, Iran had limited its nuclear program in return for relief from economic sanctions.

"I'm not convinced we're going to get there. ... I don't think the Supreme Leader of Iran wants to cut a deal," Moore told the Aspen Security Forum in Colorado.

Still, Moore cautioned: "The Iranians won't want to end the talks either, so they could run on for a bit."

Since then-U.S. President Donald Trump pulled Washington out of the deal and reimposed sanctions against Tehran in 2018, Iran has breached many of the deal's limits on its nuclear activities. It is enriching uranium to close to weapons-grade.

Western powers warn Iran is getting closer to being able to sprint toward making a nuclear bomb. Iran denies they want to do that.

U.S. President Joe Biden's administration has sought to revive the agreement. But U.S., British and French diplomats have all placed the onus on Iran for failing to bring back the nuclear agreement after more than a year of negotiations.

"I think the deal is absolutely on the table. And the European powers and the (U.S.) administration here are 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.

Iran has characterized the nuclear talks as positive and has blamed the United States for failing to provide guarantees that a new U.S. administration would not again abandon the deal as Trump had done.

Speaking later in the day at the forum, Israel's defense minister Benny Gantz said Israel had the military capability to prevent Iran from obtaining a nuclear weapon, if it came to that as a last resort. Israel sees any future Iranian nuclear weapons capability as an existential threat.

"Should we be able to conduct military operation to prevent it, if needed? The answer is yes. Are we building the ability? Yes. Should we use it as a last (resort)? Yes. And I hope that we will get United States' support," Gantz said.

Bahrain's Undersecretary of Political Affairs Sheikh Abdulla bin Ahmed bin Abdulla Al Khalifa declined to directly answer a question about whether his country might participate in pre-emptive military action against Iran's nuclear program.

But when asked whether it would be fair to interpret his answer as "an ambiguous maybe," he quipped: "Fair enough."

Bahrain is home to the U.S. Navy's Fifth Fleet headquarters, which oversees American naval operations in the Middle East.

The pact seemed near revival in March but talks were thrown into disarray partly over whether the United States might remove the Islamic Revolutionary Guard Corps (IRGC), which controls elite armed and intelligence forces that Washington accuses of a global terrorist campaign, from its Foreign Terrorist Organization list.

Biden's administration has made clear it has no plan to drop the IRGC from the list, a step that would have limited practical effect but which would anger many U.S. lawmakers.