Officials from world powers and Iran were set to meet in the Austrian capital for the first time since March, when negotiations, which began in 2021 to reintegrate the United States into the agreement, stalled.

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.

Britain, China, France, Germany, Iran, Russia and the United States signed the JCPOA in July 2015. Delegations from all will partake in Thursday's talks, but officials from the US and Iran are not expected to meet face to face.

The Joint Comprehensive Plan of Action 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 former president 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 Iran's programme was "moving ahead very, very fast" and "growing in ambition and capacity".

- Cautious optimism -

Ahead of Thursday's talks, officials expressed cautious optimism, all the while cautioning that the parties remained far apart on key issues.

The head of US delegation, Rob Malley, and the head of Tehran's delegation, Ali Bagheri, said on Twitter ahead of the talks that they were coming in good faith but put the onus on each other.

Analysts meanwhile said reviving the JCPOA remained the best option.

"The last thing the United States needs is a nuclear crisis with Iran that could easily escalate to a broader regional conflict," Suzanne DiMaggio, a senior fellow at the Carnegie Endowment for International Peace, said in a statement.

Ellie Geranmayeh, an analyst at the European Council on Foreign Relations (ECFR), said that "at the end of the day, Tehran and Washington know the alternatives to a JCPOA collapse are terrible."

"This is unlikely to be a meeting that resolves the outstanding issues" but "it could create the breakthrough necessary to push the talks towards a finishing line rather than a collapse," she said.

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