Iran and world powers will be under intense pressure at their third meeting in five weeks to secure a landmark breakthrough in their decade-old nuclear standoff.
The stakes are high as negotiators resume talks in Geneva from Wednesday, 10 days after the end of a gruelling encounter in the same Swiss city when a deal came tantalisingly close.
Failure to get an accord, or getting one seen as too lenient on Iran, will make it even harder for US President Barack Obama to dissuade Congress from applying more sanctions.
Israel, widely assumed to have a formidable nuclear arsenal itself and which has refused to rule out bombing Iran, has said it will not be bound to any document it sees as weak.
Iranian President Hassan Rouhani risks losing the backing of supreme leader Ayatollah Ali Khamenei if the moderate's "charm offensive" with the West since taking office in August fails to bear fruit -- and soon.
"So far Iranian discipline in supporting Rouhani has been impressive and unprecedented," said Trita Parsi, author and president of the National Iranian American Council.
"But if Rouhani is not getting anywhere the conservatives are going to make a strong comeback, putting them in a strong position to argue he has failed and that they should go back to old policies," he told AFP.
Undermining Iran's insistence that its programme is peaceful has been its steady expansion of its capacity to enrich uranium, which has civilian uses but which also can go in a bomb.
Iran already has enough low- and medium-enriched uranium for several bombs if it were to use its 19,000 centrifuges to process it to weapons-grade.
At present the United Nations atomic watchdog would detect any such attempt, but the fear is that soon this may no longer be the case as Iran adds more and more centrifuges, including modern ones that enrich much faster.
Another worry is the reactor being built at Arak, which theoretically could provide Iran with plutonium, an alternative to highly enriched uranium in a bomb.
Israeli and US sceptics complain that Iran consistently pretends to want a deal while using the time to install more equipment and enrich more uranium.
But the International Atomic Energy Agency said last week that since Rouhani took office, Iran has all but stopped expanding its programme. Observers called it a clear sign Khamenei wants a deal this time.
