Iran nuclear talks: We must 'seize moment', says Hague
UK Foreign Secretary William Hague urged negotiators to "seize the moment", on a third, unscheduled day of talks on Iran's nuclear programme.
Mr Hague hailed "good progress" in Geneva but said there was uncertainty about whether a deal could be reached.
An Iranian official said if a deal was not reached on Saturday, outstanding issues would carry over to a new round.
Under a possible deal, Iran could freeze expansion of nuclear activity for limited sanctions relief.
The Geneva talks involve Iran and the P5+1: the US, Russia, Britain, France and China as permanent UN Security Council members, plus Germany.
All are represented by their foreign ministers.
The talks have continued all day Saturday.
Michael Mann, a spokesman for top EU diplomat Catherine Ashton, said: "There has clearly been progress but like in all these things it's always a bit more complicated than people think, so we're working very hard and intensively to try and move things forward."
Speaking to the BBC's Middle East editor Jeremy Bowen, Iranian Foreign Minister Mohammad Javad Zarif said he was more cautious about a deal than a couple of days ago, but that a deal was coming.
Mr Zarif also said he had a message for US President Barack Obama - that a deal was attainable but that the window of opportunity would not be open indefinitely.
Mr Hague said that momentum had built up in the negotiations and there was a very different atmosphere from that of just a few months ago.
"There is now a real concentration on these negotiations so we have to do everything we can to seize the moment and seize the opportunity to reach a deal that has eluded the world," he said.
But he added that it was too early to say whether the talks would reach a successful conclusion.
French Foreign Minister Laurent Fabius also said there was "no certainty" of a deal as there were "some points with which we are not satisfied".
One sticking point was whether Iran should suspend operations at its Arak research reactor - a possible producer of weapons-grade plutonium - during negotiations, Mr Fabius told French radio.
Tehran was also resisting demands to downgrade its stockpile of uranium enriched to 20%.
"How can we bring that stock down to 5%, which is a lot less dangerous?" the French foreign minister asked.
Iranian media cited "splits" in the world powers.
"French Foreign Minister Laurent Fabius has obstructed the reaching of a deal," said Tabnak news agency, while Isna news agency quoted Mr Zarif as saying there were "differences of opinion" among the P5+1.
Iranian Deputy Foreign Minister Abbas Araqchi, who is at the negotiations, said Tehran and the world powers remained divided.
"The negotiations will not go on tomorrow. Either they end tonight or there is another round."
US Secretary of State John Kerry had met Mr Zarif and Lady 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.
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.
The West suspects Iran's uranium enrichment programme is a step towards building nuclear weapons - a charge Iran strongly denies.
Mr 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.
Defence analysts have speculated the Israelis may take military action to disrupt Iran's programme.
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.