John Kerry says no Iran nuclear deal yet
US Secretary of State John Kerry has stressed that no agreement has yet been reached on Iran's nuclear programme and that "important gaps" had to be closed.
He has joined the UK, French and German foreign ministers for unscheduled talks with Iranian representatives in Geneva.
Under a deal that has been floated Iran could freeze expansion of its nuclear activity for limited sanctions relief.
Israel's PM said he "utterly rejected" such a deal and that his country would not be obliged to abide by it.
The West has suspected Iran's uranium enrichment programme is a step towards building nuclear weapons - a charge Iran strongly denies.
The BBC's James Reynolds, in Geneva, says that for the first time in years, talks about Iran's nuclear programme are moving fast. 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 still remains uncertain.
Mr Kerry interrupted the itinerary of his tour to the Middle East and North Africa to fly to Geneva after meeting Israeli Prime Minister Benjamin Netanyahu in Jerusalem. The US state department also said that Mr Kerry would not now make planned stops in Algeria or Morocco.
Mr Kerry is holding a series of meetings in Geneva, including a trilateral with top EU diplomat Catherine Ashton and Iranian Foreign Minister Mohammad Javad Zarif.
On arrival Mr Kerry said: "I am delighted to be here at the invitation of Catherine Ashton to try to work with colleagues to see if we can narrow some differences.
"There are still some things on the table that are unresolved. I want to emphasise there is not an agreement at this point.
"[We] hope to try to narrow the differences but no-one should mistake that there are important gaps to be closed."
The Geneva talks involve the P5+1 - the five 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.
Russian Foreign Minister Sergei Lavrov is not scheduled to attend the talks but Deputy Foreign Minister Sergei Ryabkov is in the city.
Mr Fabius said there had been progress, but "nothing is hard and fast yet".
A spokesperson for David Cameron said the UK's prime minister had discussed the situation with French President Francois Hollande.
"They agreed that there is a real opportunity to make significant progress and we should do all we can to seize it," the spokesperson said.
Friday prayer leaders across Iran have urged the public to support the nuclear negotiating team, the Irna news agency reports.
The imam in Qom, Hojjat ol-Eslam Seyyed Mohammad Saidi, was quoted as telling worshippers: "The Islamic Iran under the leadership of the eminent leader supports talks with Westerners, but from a position of strength and with dignity."
Mr Zarif said on Friday there was "the general outline of an agreement".
He said: "We have now entered the very difficult and sensitive phase of editing the text that will be published, should the talks reach an agreement soon."
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.
Mr Netanyahu said agreeing the deal would be a serious mistake.
He said: "Everything [Iran] wanted, they wanted relief of sanctions after years of a gruelling sanctions regime, they got that. They are paying nothing because they are not reducing in any way their nuclear enrichment capability.
"Iran got the deal of the century and the international community got a bad deal, this is a very bad deal. Israel utterly rejects it.
"Israel is not obliged by this agreement and Israel will do everything it needs to do to defend itself and the security of its people."
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.