The US Secretary of State John Kerry said important issues still "unresolved".

According to reports, 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.

Source said 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.