Iran and the US will begin sharing ideas in Vienna about how to revive the 2015 nuclear agreement after weeks of failed starts and back-channel exchanges. Restoring the nuclear deal would be a significant development, coming nearly three years after President Donald J. Trump canceled it, which could help to break the ice between the two countries.

Officials said the complicated diplomatic dance is being discussed in which US sanctions will be removed in exchange for Iran cutting back on nuclear fuel production and allowing foreign inspectors full access to its facilities. It could take place before the Iranian presidential election in June. According to American and European officials, if even a pre-election agreement accepted by Iran's supreme leader, Ayatollah Ali Khamenei, could cement the new Iranian government.

The change came six weeks after the US proposed to join European countries in what would be the first significant diplomacy with Iran. The delay seemed to represent infighting in Iran between elements of the government desperate to end the crippling sanctions and hardliners in the military and among the clerics who have requested compensation for the harm caused by Mr. Trump's decision to withdraw from the Obama-era agreement.

Although President Biden consistently claimed during his 2020 presidential campaign that restoring the agreement was the best way to limit Iran's ambitions, his top national security advisors have also stated that restoration alone is insufficient. The next Iran agreement, according to Secretary of State Antony J. Blinken, must be "longer and stronger," implying that it must last past 2030 when many previous fuel-production agreement’s restraints will expire.

A new agreement will have to include restrictions on Iran's missile capabilities, its support for militant groups, and assistance to the Syrian government. Jalina Porter, the State Department's deputy spokeswoman, told reporters on Friday that "this is a safe first move forward."

The focus of the talks in Vienna, according to Ms. Porter, would be on "the nuclear steps that Iran would need to take to return to compliance with the terms of the Joint Comprehensive Plan of Action." That will be followed by a discussion of “the sanctions relief measures that the US will need to take to return to enforcement, as well,” she said, acknowledging that the US is still in breach of the agreement.

Ms. Porter provided no additional information, including whether Iran's special envoy, Robert Malley, would join the negotiations. Mr. Malley, who has known Mr. Blinken since they were in high school in Paris, was a key figure in the Iran talks that resulted in a settlement in 2015. Many of the key American players in those talks have been reassembled by Mr. Biden. Wendy R. Sherman, Mr. Blinken's incoming deputy who is pending Senate confirmation, was the lead day-to-day negotiator. Jake Sullivan and William J. Burns, the two men who started secret talks with Iran eight years ago, are now Mr. Biden's national security advisor and C.I.A. Director. 

All are well-versed in the inner workings of Iran's nuclear program, as well as the flaws in the 2015 agreement. It is far from obvious that they will be able to reach a better agreement. If one of the many candidates with deep links to the Islamic Revolutionary Guards Corps, which intelligence officials believe is in charge of the military side of Iran's nuclear program, wins the election, as predicted.

Officials from the United States have confirmed that they are willing to meet with Iranian officials directly, but the Iranian government has insisted on operating through the Europeans. It's unclear what the Iranians would propose in Vienna, given that they've taken multiple positions in recent weeks — first offering reparations, then proposing some sort of exchange of good-will gestures.

It's difficult to time the commitments to get both countries back into the bargain, partially because both Washington and Tehran have insisted on the other side making the first move. “I am optimistic,” said Ali Akbar Salehi, the head of Iran's Atomic Energy Organization, in a conversation on Clubhouse, an audio-chat app. Mr. Salehi, who wrote his dissertation on fast-neutron reactors at the Massachusetts Institute of Technology before the Iranian uprising found out that all of Iran's attempts to refine nuclear fuel at higher levels are reversed in a matter of a few months. These actions, he said, have granted Iran bargaining power, adding, "You can't negotiate unless you have power."

Mr. Salehi, like Iran's foreign minister, Mohammad Javad Zarif, was a key figure in the 2015 talks, saying in a Clubhouse meeting on Wednesday that "we are ready to choreograph steps" with the US. “We are not opposed to temporary measures,” he said, “but we believe that a long-term solution is more productive.”

Republicans in Congress, as well as some influential Democrats, who were opposed to the 2015 agreement, are already accusing the Biden administration of easing sanctions on Iran. Mr. Biden must be cautious not to give the impression that he is caving into Iranian demands in the Senate. After Mr. Trump reneged on his predecessor's offer, Iran has lost faith in the United States.

According to R. Nicholas Burns, who attempted the first nuclear talks with Iran as a senior State Department official under President George W. Bush. “As allies first mobilized to stop Iran's nuclear weapons aspirations in 2005-2008,” the Iranians were equally stymied by domestic arguments about how and when to negotiate”.

A senior Biden administration official claimed that in this situation, the US would not attempt to maintain any nuclear-related sanctions as leverage, stating that the Trump administration's "maximum pressure" strategy against Iran had failed.

Following the meeting on Friday, the European Union released a chairman's statement announcing the talks in Vienna " to specifically recognize sanctions lifting and nuclear implementation measures." ” Both sides, including Russia and China, “reiterated their dedication to the J.C.P.O.A.'s preservation. Iran has taken a narrow view of the deal's inspection provisions, refusing to address questions from the International Atomic Energy Agency regarding radioactive particles discovered at sites that Tehran has never announced as part of its nuke program.

Even if the current system is restored, the Biden administration will be faced with the issues left unresolved by the 2015 agreement. What kind of constraints could be placed on Iran after 2030, when the country will be free to export as much nuclear fuel as it wanted under the shattered deal? And how will the international community rein in Iran's missile programs and military support for Hezbollah, Hamas, and Shiite militias in the Middle East, as well as Syrian President Bashar al-Assad?

“The feeling is that we are on the right track,” Russia's ambassador to international organizations in Vienna, Mikhail Ulyanov, said, “but the path ahead will not be easy and will require intensive efforts.” The stakeholders will be prepared."