European intermediaries began shuttling between Iranian and US officials in Vienna on Tuesday as they sought to bring both countries back into full compliance with the 2015 nuclear deal that Washington abandoned three years ago.
Iran has steadily overstepped the accord's limits on its nuclear programme in response to Washington's withdrawal from the accord in 2018 and its reimposition of sanctions that have crippled the Islamic Republic's economy.
While Tehran has repeatedly rebuffed "direct and indirect negotiations" with its old foe, Washington said on Monday it expected the talks to be difficult. Neither side expected any early breakthrough.
"We are confident that we are on the right track, and if America's will, seriousness and honesty is proven, it could be a good sign for a better future for this agreement," Iranian government spokesman Ali Rabiei told reporters.
The remaining parties to the deal will first meet at a Viennese hotel for preparatory talks amid snowy conditions in the Austrian capital, where the pact was originally reached in 2015.
Officials from Britain, France and Germany, will act as intermediaries between Iran and the United States, shuttling between both delegations. Russia and China, the other parties to the 2015 pact, are also attending.
The US delegation, headed by special envoy Rob Malley, will be based in a nearby hotel.
President Joe Biden's administration wants to revive the accord but has said this requires negotiations. Tehran has dismissed any direct engagement for now in talks with Washington about both sides resuming compliance with the deal.
Under the 2015 accord, US and other economic sanctions on Tehran were removed in return for curbs on Iran's nuclear programme to make it harder to develop a nuclear weapon - an ambition Tehran denies.
"This is going to involve discussions about identifying the steps that the US has to take and identifying the steps that Iran is going to have to take," Malley told NPR radio on Tuesday morning. "Because they've been increasingly in noncompliance with their nuclear commitments."
Highlighting the difficulties of getting a breakthrough, Majid Takht-Ravanchi, Iran's envoy to the United Nations and a former nuclear negotiator, put the onus firmly on the United States.
"The US has so far failed to honor @POTUS campaign promise to rejoin the JCPOA (Joint Comprehensive Plan of Action). So this opportunity shouldn't be wasted," he said on Twitter. "If US lifts all sanctions, Iran will then cease all remedial measures."
Iran's Supreme Leader Ayatollah Ali Khamenei, who has the final say on all state matters, has opposed any gradual easing of sanctions.
An EU official said working groups would be created with the aim of marrying lists of sanctions that the United States could lift and nuclear obligations that Iran should meet.
Diplomats said Tuesday's talks could continue for several days to resolve some of the easier issues before resuming next week. The aim is some form of a deal ahead of June's Iranian presidential election, the official said, although Iranian and US officials have said there is no rush.
The Biden administration has also said it will seek to build a "longer and stronger agreement" that would deal with other issues, including Iran's nuclear programme after the existing deal's existing terms begin to expire, Tehran's development of ballistic missiles, and its support for proxy forces across the Middle East.
It is not clear that Iran would engage in such discussions.
"What we would pursue is, first of all, a longer agreement ... it would be better, as in any arms control agreement, to see whether we could get a follow-on deal that extends the timelines," Malley said, adding that he wanted talks about all other issues.
"But we're much better off talking about all of that if we could at least put the current nuclear issue to the side and not have to worry every day about what the latest Iranian announcement will be," he said.