US Secretary of State John Kerry and his counterparts from other world powers have gathered in Geneva hoping to put the finishing touches to a landmark nuclear deal with Iran.
It was their second attempt in the same Swiss city in two weeks to nail down a deal curbing Iran's disputed nuclear program in exchange for limited sanctions relief.
Iranian President Hassan Rouhani's election in June has created big hopes that the standoff over Iran's nuclear work can be resolved after a decade of failed diplomatic initiatives and rising tensions.
The risks posed by failure are high - further nuclear expansion by Iran, more painful sanctions and the possibility of Israeli and even US military action.
Kerry decided to go back "in light of the progress being made" after three days of negotiations in Geneva and "with the hope that an agreement will be reached", State Department spokesperson Marie Harf said on Friday.
Due to join Kerry in Switzerland were Chinese Foreign Minister Wang Yi as well as Britain's and Germany's envoys William Hague and Guido Westerwelle.
France's Laurent Fabius arrived early on Saturday and Russia's Sergei Lavrov on Friday.
Representing Iran was Foreign Minister Mohammad Jarad Zarif.
The talks are chaired by EU foreign policy chief Catherine Ashton, who held talks with different parties until the early hours of Saturday morning.
The Chinese foreign ministry said the talks were "entering their final phase" and a French diplomatic source said: "It's the homestretch but preceding negotiations have taught us prudence."
"For the first time in many years" there was a "real opportunity" for a deal, a Russian statement said after Lavrov and Zarif met late on Friday.
"I want an accord, but a solid accord," said Fabius, who secured a hardening of the P5+1's draft proposal at the last talks.
The talks involve the permanent five UN Security Council members - the United States, Russia, China, Britain and France - plus Germany.
Iran says its nuclear program is peaceful but many in the international community suspect it is aimed at acquiring nuclear weapons.
"Not all the questions are entirely dealt with. We hope to get a result during negotiations this morning," Iranian news agency IRNA quoted an informed source as saying.
The six powers want Iran to stop spinning, for six months initially, some of its many thousand centrifuges enriching uranium to levels close to weapons-grade.
They also want Tehran to stop construction work at Arak 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 unlocking several billion dollars in oil revenues and easing some trade restrictions.
This "first phase" deal would build trust and ease tensions while negotiators push on for a final accord that ends once and for all fears that Tehran will acquire an atomic bomb.
A major sticking point however has been Iran's demand - as expressed by supreme leader Ayatollah Ali Khamenei this week - for recognition of its "right" to enrich uranium.
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