Iran and the United States are struggling to overcome divisions on three major issues in indirect talks on revival of a 2015 nuclear deal while months of negotiations have entered a crucial stage.
A senior EU official shuttling between the parties said on August that a "final" offer was proposed and a response was expected within weeks.
While Tehran and Washington are set on pursuing diplomacy, the highly contentious sticking points are:
Iran insists the nuclear pact can only be salvaged if the International Atomic Energy Agency (IAEA) drops its claims about Tehran's nuclear work. Washington and other Western powers view Tehran's demand as outside the scope of reviving the deal.
In June, the UN nuclear watchdog's 35-nation Board of Governors overwhelmingly passed a resolution, drafted by the United States, France, Britain and Germany, which criticised Iran for failing to explain uranium traces found at three undeclared sites.
Iran reacted by further expanding its underground uranium enrichment by installing cascades of more efficient advanced centrifuges and also by removing essentially all the IAEA's monitoring equipment installed under the 2015 deal, a move described by the IAEA chief Rafael Grossi as potentially a "fatal blow" to reviving the agreement.
The IAEA has not had access to the data collected by such cameras, which remains with Iran, for more than a year. Grossi said more than 40 IAEA cameras would keep operating as part of the core monitoring in Iran that predates the 2015 deal.
Western powers are increasingly worried Iran is getting closer to being able to sprint towards making a nuclear bomb. Iran denies any such ambition.
Tehran seeks guarantees that "no US administration" will renege on a revived pact.
But President Joe Biden cannot promise this because the nuclear deal is a non-binding political understanding, not a legally-binding treaty.
His predecessor, President Donald Trump, withdrew Washington from the deal in 2018 and reimposed sanctions on Iran, which in 2015 had agreed to curb its nuclear programme in return for relief from US, EU and UN sanctions.
The pact, negotiated under former US President Barack Obama, was not a treaty because there was no way the Democratic president could have secured the approval of the US Senate.
Many Republican senators detest the nuclear pact and even some Democrats oppose it.
Tehran has proposed some solutions, two Iranian officials said, such as financial penalties for Western companies that may end their contracts with Tehran if Washington ditches the deal again.
Iran's elite army
The broad outline of the revived deal was essentially agreed in March after 11 months of talks in Vienna.
But then talks broke down, largely due to Tehran's demand that Washington remove its elite Revolutionary Guards Corps (IRGC) from the US Foreign Terrorist Organisation (FTO) list and Washington's refusal to do so.
The IRGC is Iran's most powerful military force with political clout and an industrial empire, which reports directly to Iran's Supreme leader Ayatollah Ali Khamenei.
In June, one Iranian and one European official said the demand had been taken off the table to give diplomacy a chance. Several sources told Reuters that Iran had agreed to discuss the matter once the 2015 pact is revived, but in return has asked for removal of sanctions on some economic units of the Guards.
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