Foreign ministers from world powers struggled Saturday to nail down a landmark nuclear deal with Iran, with US Secretary of State John Kerry announcing his imminent departure and Iran's chief negotiator downbeat.
As talks in Geneva went late into an unscheduled fourth day, Kerry's spokesman said Washington's top diplomat would be flying to London on Sunday morning -- presumably with or without a deal.
Iranian chief negotiator Abbas Araqchi said he doubted that Tehran and the P5+1 world powers -- the United States, Britain, France, China, Russia and Germany -- could reach an accord by the end of Saturday.
"Intense and difficult negotiations are under way and it is not clear whether we can reach an agreement tonight," Fars news agency quoted Araqchi as saying.
The talks, mostly between Iranian Foreign Minister Mohammad Javad Zarif and P5+1 chief negotiator Catherine Ashton, are aimed at securing a freeze on parts of Iran's nuclear programme in return for limited sanctions relief.
The arrival of Kerry and other P5+1 foreign ministers late Friday and on Saturday had raised hopes, after three long days of intense negotiations among lower-level officials, that a breakthrough was in sight.
However the talks continued to drag on inside the smart Geneva hotel late Saturday.
"We have now entered a very difficult stage," Zarif told state television.
He insisted he would not bow to "excessive demands", without detailing the obstacles.
British Foreign Secretary William Hague said on his arrival that the talks "remain very difficult" and that "we are not here because things are necessarily finished".
Late on Saturday, Kerry went into a three-way meeting with Ashton and Zarif for the second time, a US official said following a meeting among the powers' foreign ministers.
Two weeks ago, the ministers had jetted in seeking to sign on the dotted line, only to fail as cracks appeared among the P5+1 nations -- fissures that officials say are now repaired.
But a second fruitless effort in Geneva in as many weeks would not only be an diplomatically embarrassing.
If there is no deal, or at least an agreement to meet again soon and keep the diplomatic momentum going, the standoff could enter a new, potentially dangerous phase.
Since being elected in June, Iranian President Hassan Rouhani has raised big hopes that, after a decade of rising tensions over Tehran's nuclear programme, a solution might be within reach.
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Devil in the detail
But if his diplomatic push fails to bear fruit, Tehran could resume its expansion of nuclear activities, leading to ever more painful sanctions -- and possible military action by Israel or the United States.
Mark Hibbs, an analyst from the Carnegie Endowment for International Peace, said Kerry's imminent departure might not necessarily be a bad sign, however.
Kerry leaving "might set a deadline and focus people's minds, especially if things this afternoon are bogging down in the details," Hibbs told AFP.
Iran insists its nuclear programme is peaceful, but has failed to allay the international community's suspicions it is aimed at acquiring atomic weapons.
The six powers want Iran to stop enriching uranium to a fissile purity of 20 percent, close to weapons-grade, but while allowing it to continue enrichment to lower levels. That would be a step back from successive UN Security Council resolutions that have called for Iran to halt all uranium enrichment.
The powers also want Tehran to stop construction on a new reactor at Arak and to grant the International Atomic Energy Agency more intrusive inspection rights.
A hard sell
In return they are offering Iran minor and "reversible" relief from painful sanctions, including unlocking several billion dollars in oil revenues and easing some trade restrictions.
This "first phase" interim deal is meant to build trust and ease tensions while negotiators push on for a final accord to end once and for all fears that Tehran will acquire an atomic bomb.
A major sticking point has been Iran's demand -- again expressed by supreme leader Ayatollah Ali Khamenei this week -- that the powers formally recognise it has a "right" to enrich uranium.
Getting an agreement palatable to hardliners in the United States and in the Islamic republic -- as well as in Israel, which is not party to the talks -- is tough.
Israel's Haaretz daily reported that over the last three days, Intelligence Affairs Minister Yuval Steinitz spoke by phone with two of the P5+1's foreign ministers to press Israel's concerns.
In Washington there is a push by lawmakers to ignore President Barack Obama's pleas and pass yet more sanctions on Iran if there is no deal -- or one seen as too soft.