A compromise appears to be close at hand in the latest round of negotiations over Iran's nuclear programme, but key differences remain, diplomatic sources in Geneva have said.
The deal seems to hinge on Iran's stated right to enrich uranium within Iran - something which has been explicitly banned by past UN Security Council declarations.
But Al Jazeera's Jonah Hull, reporting from Geneva, said that leaks coming from negotiating teams imply "a diplomatic fudge" may be on the table to work around previous resolutions, while allowing all parties to please domestic audiences.
Our correspondent said, however, that a final deal may remain elusive.
"The language coming out of the delegations as they arrive is cautionary," he said.
We're not here because things are necessarily finished. We're here because they are difficult.
One of the key sticking points holding up any deal is likely to be the fate of Iran's heavy water reactor at Arak, which is due to come online next year.
An early morning diplomatic flurry saw US secretary of state John Kerry arrive in Geneva and hold talks with Russian Foreign Minister Sergey Lavrov. Iranian officials were in meetings with European Union foreign policy chief Catherine Ashton.
China's Wang Yi is on his way to Geneva. The talks "have reached their final moment", Chinese foreign ministry spokesman Hong Lei said, as the foreign minister left Beijing early on Saturday morning.
However, Germany's top diplomat, Guido Westerwelle, emphasised that the decade-old standoff had not yet been resolved.
"It's not a done deal," he told reporters. "There's a realistic chance, but there's a lot of work to do."
British Foreign Minister William Hague reiterated that tough negotiations continued.
"We're not here because things are necessarily finished. We're here because they are difficult," he said. There remain "narrow gaps, but important gaps" between negotiators' positions, he added.
Hassan Rouhani's election to Iran's presidency in June created hopes that the deadlock over the country's nuclear work could be resolved after a decade of failed diplomatic initiatives and rising tensions.
"We are close to a deal but still differences over two-three issues remain," said Iranian Deputy Foreign Minister Abbar Araqchi.
Risks of failure
The risks posed by failure are high: further nuclear expansion by Iran, more painful sanctions and the possibility of Israeli or even US military action.
Iran says its nuclear programme is peaceful but many in the international community suspect it is aimed at developing nuclear weapons.
The P5+1 powers - the permanent members of the Security Council and Germany - want Iran to stop, for six months initially, some of its many thousand centrifuges from enriching uranium to levels close to weapons-grade.
They also want a halt in construction at the Arak reactor and to grant the International Atomic Energy Agency more intrusive inspection rights.
In return, they are offering Iran minor and reversible relief from painful sanctions including access to several billion dollars in oil revenues and easing some trade restrictions.
This "first phase" deal would aim to build trust and ease tensions while negotiators push on for a final accord that ends once and for all fears that Iran will get an atomic bomb, and banish the spectre of a devastating new Middle East war.