Officials said that an accord was close but that a number of issues remained which need to be ironed out in an accord that would see Iran limit its nuclear work and win sanctions relief.
"It's the homestretch but preceding negotiations have taught us prudence," a French diplomat said. German Foreign Minister Guido Westerwelle said there was "still a lot of work to do."
Since being elected in June President Hassan Rouhani has raised big hopes that after a decade of rising tensions over Iran's nuclear programme a solution might be within reach.
The risks posed by failure are high, Tehran could resume the expansion of its nuclear activities, leading to more painful sanctions and even Israeli and possibly US military action.
The Geneva talks are the third round since Rouhani took office with the permanent five UN Security Council members, the United States, Russia, China, Britain and France, plus Germany, known as the P5+1.
Kerry decided to return to Geneva "in light of the progress being made" in the current round underway among political directors since Wednesday "with the hope that an agreement will be reached," the State Department said.
Joining Kerry were Chinese Foreign Minister Wang Yi as well as Britain, Germany and France's envoys William Hague, Westerwelle and Laurent Fabius. Russia's Sergei Lavrov arrived on yesterday.
Representing Iran was US-educated Foreign Minister Mohammad Javad Zarif. The talks are chaired by EU Foreign Policy Chief Catherine Ashton. Kerry held a number of bilateral meetings today.
China's Foreign Ministry said that the talks were "entering their final phase" and Lavrov said that "for the first time in many years" there was a "real opportunity".
The Iranians though sounded a note of caution, with negotiator Abbas Araqchi saying there were still "one or two points of difference".
"However, the two sides are getting close to an agreement," Araqchi told a news agency.


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