Iran has agreed to resume talks on November 29 with world powers after a five-month gap to salvage the agreement in which it promised to scale back nuclear work drastically in return for economic relief.

Much has changed since the talks broke off in June, notably Iran's election of an ultraconservative president, Ebrahim Raisi.

During the break, Iran has kept pursuing its nuclear work, leading even Western supporters of the 2015 accord to warn that the deal could become useless due to Tehran's advances.

"The Biden administration has to walk a fine line between demonstrating to Iran that Tehran will benefit from sanctions relief if the deal is restored, while not giving in to Iranian leverage," said Kelsey Davenport, director for nonproliferation policy at the Arms Control Association.

"The United States cannot reward Iran for continuing to violate the nuclear deal," she said, while adding that the Biden administration also needs to show "concrete and immediate benefits."

Biden entered the talks in Vienna -- held indirectly, with Iran refusing direct meetings with US envoy Rob Malley -- in hopes of a quick revival of the agreement from which former president Donald Trump withdrew the United States.

Trump slapped sweeping sanctions on Iran, including a unilateral ban on its oil exports, leading Iran to move away from its commitments.

- Call for assurances -

In one key point of friction, Iran is seeking a lifting of all sanctions while the Biden administration says the only measures on the table are those imposed by Trump over the nuclear program when exiting the deal.

But the task is not simple as the Trump administration in its final months duplicated many sanctions on Iran, so measures taken over nuclear work are also in force over other concerns.

Iran, for its part, wants a guarantee that the United States will maintain its commitments -- an unlikely promise for Biden, whose rivals in Trump's Republican Party have made no secret that they would shift course if they win back the White House in 2024.

Biden, nonetheless, appeared to hint at such a promise in a joint statement Saturday on the sidelines of a Group of 20 summit in Rome with the leaders of Britain, France and Germany -- nations that, along with Russia and China, remain part of the nuclear deal, formally known as the Joint Comprehensive Plan of Action.

The three European leaders said they "welcome President Biden's clearly demonstrated commitment to return the US to full compliance with the JCPOA and to stay in full compliance, so long as Iran does the same."

Russia's ambassador to international organizations in Vienna, Mikhail Ulyanov, called Biden's pledge "a significant step towards assurances and guarantees Iran is looking for."

- War threats -

But Western nations have increasingly questioned whether Raisi, as well as Supreme Leader Ali Khamenei, are interested in maintaining the accord and will watch carefully in Vienna to see how Iran negotiates.

In a sharp shift in tone last month, Secretary of State Antony Blinken said that the United States had "all options" available as he sat next to Israeli Foreign Minister Yair Lapid, who openly warned of a military attack.

In an opinion piece that drew wide notice, Dennis Ross, a veteran Middle East adviser to Democratic presidents, said that the United States needed to keep the threat of war on the table for successful negotiations.

"If the United States wants to reduce the risk of a conflict and give diplomacy a chance to succeed, the Biden administration is going to have to restore Iran's fear of a US reaction and apply pressure far more effectively," he wrote in Foreign Policy.

Adam Schiff, a Democrat who heads the House intelligence committee, warned in an appearance this week that Congress could reimpose sanctions if Iran does not budge.

Davenport, however, said there was no military solution and that there was fatigue on sanctions, with China unlikely to cut back on its oil purchases from Iran.

If Iran will not return to compliance, Biden could instead pursue a short-term deal that includes modest sanctions relief for a freeze on sensitive activities, she said.

"The United States has other options, but none of them are good," she said.