The US State Department's point      man on negotiations with Iran, Rob 

"Our expectations are in check, but the United States welcomes EU efforts and is prepared for a good faith attempt to reach a deal," Malley said.

"It will shortly be clear if Iran is prepared for the same," he added.

EU nuclear coordinator Enrique Mora was also due in Vienna "to discuss (getting) JCPOA back to full implementation", he wrote on Twitter, referring to the 2015 deal intended to prevent Tehran developing a nuclear bomb.

Iran said it was "optimistic" about the talks as it also announced on Wednesday that it was sending a delegation led by Ali Bagheri "as part of the policy of lifting cruel sanctions against our country".

Negotiations between Iran and world powers, which began in 2021 to reintegrate Joe Biden's United States into the agreement, have been stalled since March.

In late June, Qatar hosted indirect talks between Tehran and Washington in the hope of getting the process back on track -- but those talks failed to make a breakthrough.

In a last-ditch effort, EU foreign policy chief Josep Borrell submitted a compromise proposal last month and called on the parties to accept it to avoid a "dangerous nuclear crisis".

Borrell said the draft text includes "hard-won compromises by all sides" and "addresses, in precise detail, the sanctions lifting as well as the nuclear steps needed to restore" the 2015 pact.

- 'Hope for a breakthrough' -

A Vienna-based European diplomat told AFP he welcomed "a meeting that showcases the willingness of everyone to move forward".

"It's positive, but at the same time nothing is guaranteed. We have been trying to conclude the negotiations for months," he added.

Ali Vaez of the International Crisis Group said the parties' return to Vienna means there is "some hope for a breakthrough".

But Henry Rome, an Iran specialist at American consultancy Eurasia Group, told AFP expectations were "rightfully modest" because the United States and Iran "remain far apart" key issues.

These include sanctions, Iranian demands for guarantees and the end of a probe by the UN nuclear watchdog. "Resolving each of them will be hard," said Vaez.

But "both sides benefit from keeping the prospect of a deal alive even as they both seem resigned to its eventual demise," Rome told AFP.

"Given how unattractive the alternatives to the agreement are for both sides, neither is willing to pull the plug on the negotiations," added Vaez.

Signed by Iran and six powers -- Britain, China, France, Germany, Russia and the United States -- the JCPOA aims to guarantee the civilian nature of Iran's nuclear programme in exchange for a gradual lifting of sanctions.

But following the unilateral withdrawal of the United States in 2018 under Donald Trump and the re-imposition of US sanctions, Tehran has backtracked on its obligations.

Iran subsequently exceeded the JCPOA's uranium enrichment rate of 3.67 percent, rising to 20 percent in early 2021.

It then crossed an unprecedented 60-percent threshold, getting closer to the 90 percent needed to make a bomb.

The head of the UN nuclear watchdog, Rafael Grossi, on Tuesday warned that Iran's programme was "moving ahead very, very fast" and "growing in ambition and capacity".

"When it comes to nuclear, good words will not do it. What you need to do is to be transparent and compliant and work with us," he told a press conference in New York.