Global powers upped the pressure on Iran late Sunday warning now was "the time" to strike an elusive deal curbing its nuclear ambitions, as the US said the fate of the talks hung in the balance.
US Secretary of State John Kerry stressed that after almost two years of negotiations and on the ninth day of these latest talks in Vienna "genuine progress" had been made.
But the top US diplomat said the talks still "could go either way" as he met four times throughout Sunday with his Iranian counterpart Mohammad Javad Zarif.
French Foreign Minister Laurent Fabius and the EU's foreign policy chief Federica Mogherini arrived back in the Austrian capital with essentially the same message ahead of Tuesday's deadline for a comprehensive deal curtailing Iran's nuclear programme.
"All the cards are on the table, the main question is to know whether the Iranians will accept making clear commitments on what until now has not been clarified," Fabius said.
Kerry stressed that despite progress "we are not yet where we need to be on several of the most difficult issues".
If all sides were prepared to make hard choices, then "we could get an agreement this week. But if they are not made, we will not," Kerry warned, adding that if there was "absolute intransigence" the US would walk away.
The global powers -- Britain, China, France, Germany, Russia and the United States -- are trying to pin down a deal putting a nuclear bomb out of Iran's reach in return for lifting a web of sanctions against the Islamic republic.
"The time is now... We are very close," said Mogherini, adding the atmosphere was "constructive, positive."
On one of the thorniest issues -- choreographing the nuclear steps to be taken by Iran in exchange for reciprocal sanctions relief -- a compromise may be emerging.
The deal between Iran and the P5+1 would end a standoff dating back to 2002 when dissidents first revealed undeclared nuclear facilities in Iran.
- Hard to walk away -
Officials have stressed all week that this is the endgame, and after missing several deadlines they are not planning to extend the negotiations again.
"I hope that it is clear to all parties in these final days that this is a unique chance that we have now," said German Foreign Minister Frank-Walter Steinmeier as he arrived back in Vienna.
"We have a wonderful chance after 12 years of talks finally to end a long-running conflict... which would also send out a signal for the whole region."
Russian Foreign Minister Sergei Lavrov also arrived back in the city, his ministry tweeted.
A deal would also hold out the promise of bringing Iran back into the diplomatic fold at a time of mounting unrest in the Middle East.
Zarif said in an English YouTube message that an accord could "open new horizons to address important common challenges", referring to the "growing menace of violent extremism and outright barbarism," an implicit statement on the Islamic State (IS) militant group.
On Saturday it appeared that another stumbling block to the deal -- a stalled UN probe into allegations of past efforts by Iran to develop the bomb -- may potentially be close to being resolved too.
Senior officials from the International Atomic Energy Agency (IAEA) were flying out to Iran, just days after last week's visit by the UN watchdog's chief Yukiya Amano.
"I think it would be very difficult to imagine Secretary Kerry at this point walking away, this close to the finish line," Iran expert Suzanne Maloney from the Brookings Institution told AFP.
"I just don't think there's any real likelihood that this collapses."