The US has agreed to the idea of taking part in talks involving Iran and world powers with a view to reviving a landmark nuclear deal.
Iran vowed to limit its nuclear programme in return for the lifting of economic sanctions under the 2015 deal.
But former President Donald Trump withdrew the US from the deal in 2018, leading Iran to roll back commitments.
Now the US has expressed intent to rejoin the deal under President Joe Biden.
On Thursday, the Biden administration made its firmest pledge yet to re-engage with Iran over the deal, known as the Joint Comprehensive Plan of Action (JCPOA).
State department spokesman Ned Price said the US would accept an invitation from the European Union to meet Iran for talks.
Senior EU diplomat Enrique Mora tweeted he was "ready to invite" all parties to talks, saying this was a "critical moment" for the deal.
Iran has not formally responded to the proposal.
But in a tweet, Iran's Foreign Minister Mohammad Javad Zarif suggested the country would only comply with the deal fully once US sanctions had been lifted.
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Iran has ratcheted up pressure on the Biden administration, threatening to block international inspections of its nuclear sites within days if the US does not lift sanctions.
Since the US withdrew from the deal, Iran, which says its nuclear programme is peaceful, has resumed or begun nuclear activities barred under its terms. This has heightened concern among the parties to the deal, who suspect Iran's intentions.
In response, the US and its European allies - the UK, France and Germany - have called on Iran to refrain from blocking inspections, warning jointly that the move would be "dangerous".
On Thursday, the four powers said they shared a commitment to ensuring Iran could "never develop a nuclear weapon".
This would be the first substantial diplomacy with Iran in more than four years, an initial meeting hosted by the EU to get around the stand-off over who moves first in the complex path back to the nuclear deal.
To show good faith, the Biden administration eased stringent travel restrictions on Iran's UN diplomats imposed by Mr Trump. It also reversed his effort to restore all UN sanctions on Iran.
When it comes to American sanctions, Mr Biden says he'll lift them once Iran has resumed compliance with the deal.
But Iran has demanded an end to US sanctions by the weekend, threatening to limit inspections of nuclear sites if it doesn't. The US and other Western powers have urged it against taking what they call a step backward when they're ready to move forward.
Iran's reaction to that statement was dismissive, but it's not clear how it would respond to a formal invitation for talks.
What did the deal do?
Suspicions that Iran was using its nuclear programme as a cover to develop a nuclear bomb prompted the EU, US and UN to impose sanctions in 2010.
In 2015, Iran reached a deal with six powers - the US, UK, France, China, Russia and Germany - that saw it limit its nuclear activities in return for sanctions relief.
Under the terms of the deal, Iran agreed to limit its uranium enrichment and allow international inspectors to access sites and facilities.
Why did the deal fall apart?
Mr Trump abandoned the deal in May 2018 and reinstated US sanctions. He wanted a new deal that would also curb Iran's ballistic missile programme and its involvement in regional conflicts.
Iran refused and, when the sanctions took effect, was plunged into an economic crisis.
Since sanctions were tightened in 2019, Iran has moved to stop international inspections of its nuclear sites and step up uranium enrichment in breach of the deal.
The window for saving the international nuclear deal with Iran is rapidly narrowing - and a power-struggle inside the Islamic republic between those for and against it could soon seal its fate.
Iran holds crucial presidential elections in June, and hardliners who see the deal as a humiliation want to stall its revival before the polls. The incumbent president, Hassan Rouhani, a relative moderate and champion of the deal, cannot stand again following two terms in office. Anti-deal conservatives - already dominant in parliament - hope to replace him with a figure of their own.
While in the US opposition to the deal can generally be split along Republican (against) and Democrat (for) lines, in Iran it involves a more complicated political dynamic. This is because of how Iranian public opinion has largely turned against the regime since US sanctions were re-imposed in 2018 after the administration of former President Donald Trump abandoned the deal.
On one hand, there are the hardliners who feel that Mr Rouhani's government has been too conciliatory and that a government led by themselves can extract more concessions from the US.
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This was the strategy behind the passing of a bill in December that reduces even further Iran's nuclear commitments by setting a 22 February deadline for sanctions removal by the US or have Iran bar short-notice inspections of sites by International Atomic Energy Agency (IAEA) experts.
President Rouhani has accused his hardline rivals of wanting to take credit for actually reviving the nuclear deal and even went as far as accusing them of "wanting Trump to have won US elections" so it would not happen on his watch.
On the other hand, many Iranians are not too excited about any measures that can give a new lease of life to a regime under siege by economic strangulation and whose legitimacy is questioned at home and abroad more than ever.
This public discontent has been fuelled by years of crippling US-led sanctions intensified under Donald Trump, which have led to inflation, rising unemployment and unprecedented anti-government street protests - which in turn led to more repression.
Many frustrated and disillusioned Iranians have subscribed to the "regime change" school of thought advocated by supporters of Donald Trump's "maximum pressure" policy, though the former US administration never officially endorsed such a move.
Pro-Trump sentiments voiced around the time of the US elections by some Iranian academics, political activists and even former officials - from the daughter of ex-President Hashemi Rafsanjani to the adviser of another former President, Mahmoud Ahmadinejad - continue to be expressed on Iranian social media.
Many Iranians may feel that a revival of the nuclear deal will only prolong their hardship by extending to the regime greater international acceptance, and as a result will not bother voting in the June elections. Low turnouts in Iran historically tend to favour hardliners.
Iranian hardliners have been branded "sanction merchants" by moderates, referring to their vested economic interests in maintaining an underground economy that reaps profits by evading global trading restrictions.
Abbas Akundi, a moderate government minister, said such profits amounted to some "$25bn per annum".
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