Iran and world powers were set Thursday to begin hammering out a landmark deal freezing parts of Tehran's atomic programme to ease fears of the Islamic republic getting nuclear weapons.
Diplomats said the talks might still fail, with Iran's lead negotiator echoing on Thursday comments by the supreme leader who ruled out a deal with no recognition of Iran's "right" to enrich uranium.
"No deal that does not include the right to uranium enrichment from start to finish will be accepted," Abbas Araghchi said on his Twitter account, quoted by official news agency IRNA.
"Considerable gaps remain," a senior Western diplomat said after Wednesday's initial meetings in rainy Geneva.
Another was more upbeat: "I am not saying it's in the bag but we are in a process that started well and which could lead to a deal this weekend ... We are getting to the heart of the matter."
French Foreign Minister Laurent Fabius, who at the last round 10 days ago succeeded in toughening the powers' draft proposal, said Thursday he was hoping for "solid" deal.
"This deal will only be possible if it has a firm base," Fabius told France 2 television.
"Of course Iran has the right to a civilian nuclear programme but not to an atomic bomb. This is the common position we are defending."
Numerous attempts to resolve the standoff have failed over the last decade, but the election this year of Hassan Rouhani as president has raised hopes that this time a deal can be struck.
With the country reeling from sanctions, since taking office in August Rouhani has put the brakes for the first time in years on expanding Iran's atomic activities.
The United States, Russia, China, Britain, France and Germany, the P5+1, now want Iran to suspend certain parts of its nuclear programme for several months.
In return Iran is being offered minor sanctions relief, although officials insist that core sanctions on its oil exports and banks will stay in place.
Over subsequent months a final deal would be worked out.
On Wednesday US Secretary of State John Kerry sought to reassure sceptics at home and in Israel worried that Iran would still be left with the capability to make a nuclear bomb.
"We will not allow this agreement, should it be reached ... to buy time or to allow for the acceptance of an agreement that does not properly address our core, fundamental concerns," Kerry said in Washington.
Israel though, which has refused to rule out bombing Iran and which is assumed to have nuclear weapons itself, wants a total and permanent dismantlement of Tehran's nuclear facilities.
Daryl Kimball from the Arms Control Association said that this might have been possible in 2005 when Iran had fewer than 300 centrifuges at one site.
"But it is not realistic now that Iran has 19,000 installed and 10,000 operating centrifuges at two sites," he said.
If Rouhani, meanwhile, fails to secure quick and substantial relief from the sanctions, he risks losing the support of arch-conservatives and the supreme leader, experts say.
Down to business
Iranian Foreign Minister Mohammad Jarad Zarif said on Facebook that on Thursday he would hold "serious and detailed" talks with P5+1 chief negotiator and EU foreign policy head Catherine Ashton.
"I will have a meeting with Lady Ashton, which is the beginning of negotiations to examine how to finalise the draft," the ISNA news agency quoted Zarif as saying.
This would be followed later by talks between Zarif and the political directors of all six powers chaired by Ashton.
Araqchi said that "the lost confidence must be regained", in reference to the last Geneva talks 10 days ago when French objections reportedly delayed an accord.
Now the six powers are united, diplomats insist, despite intense diplomatic efforts by Israeli Prime Minister Benjamin Netanyahu to avert what he calls a "bad deal".
On Wednesday Netanyahu took his campaign to Moscow for talks with Russian President Vladimir Putin, who said only he hoped for a "mutually acceptable" solution.
Netanyahu likened the deal to Syria only having to destroy only some of its stockpile of chemical weapons and leaving it capable of making more.
"That is more or less what's on the table in Geneva," he said.