Iran said on Monday that work to revive a 2015 nuclear deal could succeed if the US decided to meet Tehran's remaining demands, as months of talks enter what one Iranian diplomat called a "now or never" stage.
The stakes are high, as the failure of 10 months of talks would carry the risk of a regional war and the imposition of more harsh sanctions against Iran by the West.
Iran's Foreign Ministry spokesman said the remaining issues were the extent to which sanctions would be withdrawn, guarantees that the US would not leave the pact again, and questions over uranium traces found at old, undeclared sites in Iran.
All parties to the talks say progress has been made towards restoring the pact to curb Tehran’s nuclear programme in exchange for sanctions relief. The US abandoned the agreement in 2018.
But Tehran and Washington have cautioned that there are still significant differences to overcome.
"Reaching a good deal is possible … three key issues still remain to be resolved," Iran's Foreign Ministry spokesman Saeed Khatibzadeh said.
"The US and European powers have not taken political decisions on these major issues."
France's Foreign Ministry said on Monday that it was urgent to conclude the talks this week.
Iran's lead nuclear negotiator, Ali Bagheri Kani, who flew to Tehran last week for consultations about the final draft of the deal, on Monday met the EU's Enrique Mora, who co-ordinates the talks in Vienna.
Sources in Vienna said Iran had submitted new demands, while continuing to insist on existing ones, including the removal of a US terrorist organisation designation against Iran's Islamic Revolutionary Guard Corps.
"Iran's stance after Bagheri's trip to Tehran has become even more uncompromising … they now insist on removal of sanctions on the IRGC and want to open issues that had already been agreed," one source said.
Iran has said that removing the Guards from the terrorism list was under discussion. Iranian officials did not immediately respond to a request for comment on the matter.
Diplomats said the negotiations have entered a crucial stage, given Iran's uncompromising policy and the other parties' involvement in the crisis over Ukraine.
"It is now or never. If they cannot reach a deal this week, the talks will collapse forever," an Iranian diplomat said in Tehran.
The IRGC is a powerful faction in Iran that controls a business empire, elite armed forces and intelligence services.
Dozens of its commanders have high-ranking positions in hardline President Ebrahim Raisi's government.
The terrorist designation of the IRGC by Washington in 2019, which was the first time the US had formally called part of another sovereign government a terrorist group, caused further problems for Iran's sanction-hit economy.
Tehran also insists the International Atomic Energy Agency drop its claims about Tehran's nuclear work, objecting to an assertion by the UN watchdog last year that Tehran had failed to fully explain the presence of uranium traces found at undeclared sites.
"We have answered the agency's questions. But instead of closing the politically motivated case, they are using it to gain leverage in the talks," an Iranian official said in Tehran.
Iran's arch-foe Israel has pushed for a tough policy if diplomacy fails to rein in Iran's nuclear work. Tehran has warned of a "crushing" response if attacked.
In reaction to former US president Donald Trump's reimposition of sanctions after ditching the nuclear deal in 2018, Tehran has breached limits of the pact.