Crunch nuclear talks between Iran and world powers enter a make-or-break third day on Friday, with Tehran complaining of "no progress" so far on narrowing considerable differences.
This third round of talks in Geneva since President Hassan Rouhani was elected in June are seen as the biggest hope in years in resolving the decade-old standoff over Iran's nuclear programme.
Reaching an agreement palatable to sceptical hardliners both in the United States and in the Islamic republic -- and Israel -- is a big ask, however.
Failure might mean Iran resuming the expansion of its atomic activities, Washington and others adding to already painful sanctions, and possible Israeli military action.
Britain, China, France, Russia, the United States and Germany, known as the P51, want Iran to freeze for a period of six months parts of its nuclear programme.
In return Iran would get minor and, Western officials insist, "reversible" sanctions relief, including unlocking several billion dollars in oil revenues stuck in international bank accounts.
This hoped-for "first phase" deal would build trust and ease tensions while Iran and the six powers hammer out a final accord that ends once and for all fears that Tehran will get an atomic bomb.
"A first step is a very, very important deal because it means that a deal has been struck, which changes the atmosphere, changes the conversation," said Trita Parsi, author and president of the National Iranian American Council.
Netanyahu says no
Many in Israel, widely assumed to have a formidable nuclear arsenal itself, are alarmed, however, with Prime Minister Benjamin Netanyahu campaigning tirelessly against the mooted deal.
Netanyahu wants all, not part, of Iran's nuclear infrastructure dismantled, believing that the P51 will leave Iran with an ability to develop nuclear weapons.
"You are not really dismantling any capacity to make fissile material for nuclear weapons," he said in an interview in Germany's Bild, Europe's top-selling newspaper, this week.
In the United States meanwhile there is a push by lawmakers to ignore President Barack Obama's pleas and pass yet more sanctions on Iran if there is no deal -- or one seen as too soft.
Raising the pressure, US Senate Majority Leader Harry Reid said in Washington on Thursday that lawmakers would move to impose new sanctions on Iran in December.
This risks spoiling Iran's apparent newfound appetite for rapprochement with the West since Rouhani replaced the more hardline and combative Mahmoud Ahmadinejad in August.
Rouhani is under pressure to show the supreme leader Ayatollah Ali Khamenei the first fruits of his "charm offensive", and it is unclear whether the minor sanctions relief on offer is enough.
Khamenei, describing Israel as a doomed "rabid dog", said in Iran on Wednesday on the first day of the talks that he insisted "on not retreating one step from the rights of the Iranian nation".
By this he meant what he sees as the "right" to enrich uranium, something which the P51 are loath to explicitly endorse.