Talks between Iran and world powers on Tehran’s nuclear programme are to enter an unscheduled third day in Geneva.
A US State Department official said at the end of Friday’s talks that “we continued to make progress as we worked to narrow the gaps”.
But French Foreign Minister Laurent Fabius warned on French radio that there was “no certainty” of a deal.
Under a deal being floated, Iran could freeze expansion of its nuclear activity for limited sanctions relief.
“There is more work to do,” the US official told reporters after talks ended late on Friday evening.
Iranian Deputy Foreign Minister Abbas Araqchi echoed that sentiment, saying: “It was productive but still we have lots of work to do.”
US Secretary of State John Kerry had met his Iranian counterpart Mohammad Javad Zarif and top EU diplomat Catherine Ashton for five hours on Friday evening.
Mr Kerry, who interrupted the itinerary of his tour to the Middle East and North Africa to join the talks, had earlier said that there were still “important gaps” between Iran and world powers.
The West has suspected Iran’s uranium enrichment programme is a step towards building nuclear weapons – a charge Iran strongly denies.
UK, French and German foreign ministers have also joined the unscheduled talks with Iranian representatives in Geneva.
Russian Foreign Minister Sergei Lavrov and a senior Chinese diplomat are expected to join the talks on Saturday.
For the first time in years, talks about Iran’s nuclear programme are moving fast, the BBC’s James Reynolds reports from Geneva.
The negotiations have also picked up new, critical purpose, he says – a chance for the US and Iran to explore an end to their three decades of mistrust.
However, although the sides appear closer to a breakthrough than during previous talks, the outcome remains uncertain.
The Geneva talks involve the P5+1: the US, Russia, Britain, France and China as permanent UN Security Council members, plus Germany.
The French, German and UK Foreign Ministers – Laurent Fabius, Guido Westerwelle and William Hague – are also in Geneva.
Friday prayer leaders across Iran urged the public to support the country’s nuclear delegation as it negotiated “from a position of strength and with dignity”.
Mr Zarif said on Friday there was “the general outline of an agreement”.
Although details of the suggested deal have not been disclosed, it is thought to offer Iran a gradual easing of sanctions in return for a freeze on expansion of nuclear activities.
Meanwhile, US President Barack Obama telephoned Israeli Prime Minister Benjamin Netanyahu on Friday to allay fears about any deal.
A White House statement said Mr Obama had updated the Israeli leader on the talks in Geneva, and repeated his commitment to stopping Tehran getting a nuclear weapon.
Mr Netanyahu had earlier said he “utterly rejected” such a deal with Iran, its long-time enemy, and that his country would not be obliged to abide by it.
The Israeli prime minister argued the Iranians were getting sanctions relief while “not reducing in any way their nuclear enrichment capability”.
Israel, Mr Netanyahu said, would do “everything it needs to do to defend itself and the security of its people”.
Israel’s Prime Minister Benjamin Netanyahu: “Iran got the deal of the century and the international community got a bad deal”
Defence analysts have speculated the Israelis may take military action to disrupt Iran’s programme.
White House spokesman Josh Earnest said that any criticism of a deal was “premature”.
“There is no deal, but there is an opportunity here for a possible diplomatic solution, and that is exactly what [President Obama] is pursuing,” he said.
Since 2006 the UN Security Council has imposed a series of sanctions – including asset freezes and travel bans – on entities and people involved in Iran’s nuclear programme.
Separate US and EU sanctions have targeted Iran’s energy and banking sectors, crippling its oil-based economy.
The head of the UN atomic watchdog, the International Atomic Energy Agency, Yukiya Amano, is travelling to Tehran on Monday to meet senior Iranian figures.