Representatives of the world powers that signed the 2015 Iran nuclear deal are headed to Vienna on Tuesday to save the landmark accord.
The United States is also due to start indirect talks with Tehran. President Joe Biden has said he wants to return to the landmark accord.
But the six remaining state parties need to find a way for him to lift the sanctions imposed by his predecessor and for Iran to return to the agreed limits on its nuclear programme.
Iran has said it will not meet the US face to face until that happens.
The top US officials attending the meeting in Austria will reportedly be based in a different hotel to the one hosting the meeting of the delegations from Iran and the other world powers – China, France, Germany, Russia and the UK. European officials will act as intermediaries.
“We don’t underestimate the scale of the challenges ahead,” US state department spokesman Ned Price told reporters on Monday.
“These are early days. We don’t anticipate an early or immediate breakthrough, as these discussions, we fully expect, will be difficult.”
The nuclear deal, known as the Joint Comprehensive Plan of Action (JCPOA), has been in intensive care since Donald Trump pulled the US out of it.
He said it was based on “a giant fiction that a murderous regime desired only a peaceful nuclear energy programme” and reinstated crippling economic sanctions in an attempt to compel Iran to negotiate a replacement.
Iran, which insists it does not want nuclear weapons, refused to do so and retaliated by rolling back a number of key commitments under the accord.
Since the end of the year, it has accelerated the breaches in an attempt to increase pressure on the US. They have included operating advanced centrifuges to enrich uranium, resuming enrichment to 20% concentration of the fissile U-235 isotope, and building a stockpile of that material.
The nuclear deal only allows Iran to produce and store limited quantities of uranium enriched up to 3.67% concentration, which can be used to produce fuel for commercial nuclear power plants. Uranium that is enriched to 90% or more can be used to make nuclear weapons.
Iran’s government has said the steps were taken to comply with a law passed by parliament following the assassination in November of the top nuclear scientist, Mohsen Fakhrizadeh, which Iranian officials blamed on Israel.