Critical talks with Iran to prevent the collapse of a nuclear deal have resumed in Vienna after a five-month pause.
Officials are discussing the possible return of the US to the 2015 accord, which limited Iran's nuclear activities in return for the lifting of sanctions.
Iran has violated key commitments since then-President Donald Trump pulled out in 2018 and reinstated US sanctions.
Joe Biden is willing to lift them if Iran reverses the breaches. But Iran wants the US to make the first move.
Western diplomats have warned that time is running out to negotiate a solution because of the significant advances Iran has made in its uranium enrichment programme, which is a possible pathway to a nuclear bomb.
Iran insists that its nuclear programme is entirely peaceful.
The talks between Iran and the five countries still party to what is known formally as the Joint Comprehensive Plan of Action (JCPOA) - China, France, Germany, Russia and the UK - began in the Austrian capital in April, with US representatives participating indirectly.
A senior US official told the New York Times last week that an agreement on which steps needed to be taken and when by Washington and Tehran was "largely complete" before the Iranian presidential election in June.
Ebrahim Raisi, a hardliner and strident critic of the West, won the race to succeed Hassan Rouhani, a moderate who negotiated the JCPOA with Barack Obama's administration.
Mr Raisi promised before taking office in August that he would not to let the talks drag on, but he did not agree to return to Vienna until earlier this month.
The Iranian foreign ministry has said it wants an "admission of culpability" from the US; the immediate lifting of all US sanctions; and a "guarantee" that no future US president will unilaterally abandon the deal again.
"To ensure any forthcoming agreement is ironclad, the West needs to pay a price for having failed to uphold its part of the bargain," Iran's chief nuclear negotiator, Ali Bagheri Kani, wrote in the Financial Times on Sunday.
Mr Biden's special envoy, Robert Malley, has said the US is prepared to take all of the steps necessary to come back into compliance, including lifting the sanctions imposed by the Trump administration that crippled the Iranian economy.
But he has also said the window for negotiations will not be open forever.
"If Iran thinks it can use this time to build more leverage and then come back and say they want something better it simply won't work. We and our partners won't go for it," he told the BBC on Saturday.
Mr Malley warned Iran that if it was to "try to use the negotiations as cover for an accelerated nuclear programme and drag its feet at the nuclear table", then the US would be forced to "respond in a way that is not our preference".
Israel's Prime Minister, Naftali Bennett, said on Monday that Iran was seeking "to end sanctions in exchange for almost nothing", and urged world powers not to give into its "nuclear blackmail". He has said Israel will not be bound by any deal and signalled his readiness to to take action to to stop Iran acquiring a nuclear bomb.
A major focus of the JCPOA was Iran's production of enriched uranium, which is widely used as fuel for nuclear power plants but can also be used in nuclear weapons.
Iran agreed to limits on the amount of the material that it could stockpile; the level of purity; the number and type of centrifuge machines that could be used to carry out enrichment; and the locations where it could take place.
It began gradually breaching those restrictions in 2019 in retaliation for the sanctions reinstated by Mr Trump, who called the JCPOA "defective at its core" and wanted to compel Iranian leaders to negotiate a replacement.
Iran has now amassed a stockpile of enriched uranium that is many times larger than permitted, including some that is 60% purity - a short technical step away from the 90% needed to make a weapon.
It has also installed hundreds of advance centrifuges; resumed enrichment at previously-converted underground facility; and taken steps to produce enriched uranium metal, which is a key material in nuclear bombs.
Inspectors from the International Atomic Energy Agency have also had their access to Iranian nuclear facilities significantly curtailed.
The mood music surrounding these talks is not great.
Iran's new government has dragged its feet, taking almost six months to come back to Vienna. It arrives at the table laden with new, maximalist demands. It's not going to talk about its nuclear activities, Iran says. These talks should be all about the US lifting sanctions. All of them. Immediately. Verifiably. With guarantees a future US government would not pull out of the deal.
The US and other signatories to the original nuclear deal are in turn demanding the talks pick up where they left off in June, when both sides expressed confidence an agreement was possible.
And if not, if Iran refuses to talk about its accelerated nuclear programme, then American diplomats talk of "other options", "other tools" for preventing Iran getting a nuclear weapon - a euphemism for allowing Israel to launch military or cyber attacks on Iranian facilities.
The bottom line is that Western powers do not yet know the intentions of this new government in Tehran: is it serious about negotiating a deal and agreeing the compromises that would be necessary?
Or is it playing for time to enrich more uranium, which potentially could be used in a nuclear weapon that Iran insists it does not want?
Much depends on the answer to that question.
These talks could drag on for some time, and as of now in Vienna there's more snow on the ground than optimism.