Barring a last-minute breakthrough, talks between Iran and six world powers on curbing Tehran's nuclear ambitions were set to end without a full agreement on Saturday as a split emerged between France and the other Western powers, diplomats said.
Ministers from Iran and the major powers held a series of meetings late on Saturday in a final push to hammer out the outline of a deal that would freeze parts of Iran's atomic programme in exchange for sanctions relief.
"Efforts to secure an agreement are continuing with great intensity," a Western diplomat close to the talks said.
The latest round of talks began on Thursday and US Secretary of State John Kerry unexpectedly arrived on Friday to help narrow remaining differences between Iran and the six nations.
While a deal appeared unlikely on Saturday, Western diplomats said the talks were expected to resume within a few weeks. Iranian Foreign Minister Mohammad Javad Zarif said that if there was no agreement this weekend, "the process will continue in one week or 10 days".
French Foreign Minister Laurent Fabius also said it was not clear the delegations would succeed in nailing down an acceptable interim deal that would begin to defuse fears of a stealthy Iranian advance towards nuclear arms capability.
"As I speak to you, I cannot say there is any certainty that we can conclude," Fabius told France Inter radio, saying Paris could not accept a "fool's game".
His pointed remarks hinted at a rift within the Western camp. A Western diplomat close to the negotiations said the French were trying to upstage the other powers.
"The Americans, the EU and the Iranians have been working intensively together for months on this proposal, and this is nothing more than an attempt by Fabius to insert himself into relevance late in the negotiations," the diplomat told Reuters, speaking on condition of anonymity.
In a further sign that the cordiality that reigned in the first round of talks last month and earlier this week was dissipating, Iranian Deputy Foreign Minister Abbas Araqchi told Mehr news agency that his counterparts from the six powers "need constant coordination and consultation in order to determine (their) stances".
The main sticking points appeared to include calls for a shutdown of an Iranian reactor that could eventually help to produce weapons-grade nuclear fuel, the fate of Iran's stockpile of higher-enriched uranium, and the nature and sequencing of relief from economic sanctions sought by Tehran.
Kerry said on Friday there were "some very important issues on the table that are unresolved".
He avoided media on Saturday before holding several hours of intensive talks with Zarif and European Union foreign policy chief Catherine Ashton. The three met for five hours on Friday and again late on Saturday in a final push for a deal.
Foreign ministers from four of the five permanent UN Security Council members - Britain, France, Russia and the United States - and Germany took part in Saturday's talks. China, also a permanent member, sent a deputy foreign minister, who arrived on Saturday evening.
But it is the Americans and Iranians, whose countries have not had formal diplomatic ties for more than three decades, who have the power to make or break an agreement.
The fact that any deal might be feasible after a decade of feuding highlights a striking shift in the tone of Tehran's foreign policy since the landslide election in June of the relative moderate Hassan Rouhani as president.
The powers remain concerned that Iran is continuing to amass enriched uranium not for future nuclear power stations, as Tehran says, but as potential fuel for nuclear warheads.
They are searching for a preliminary agreement that would restrain Iran's nuclear programme and make it more transparent for UN anti-proliferation inspectors. In exchange, Tehran would obtain phased, initially limited, relief from the sanctions throttling the economy of the giant OPEC state.
The goal now is to take a big first step towards resolving a dispute rife with political baggage and legal complexities, and so arrest a drift towards a major new war in the world's most volatile region.
Iran spelled out one major bone of contention. A member of its negotiating team, Majid Takt-Ravanchi, told Mehr news agency on Friday that Western powers should consider easing oil and banking sanctions during the first phase of any deal.
The powers have offered Iran access to Iranian funds frozen abroad for many years but ruled out any broad dilution of the overall sanctions regime in the early stages of an agreement.
Diplomats said that even a breakthrough this weekend would be only the start of a long confidence-building process.
But they said the arrival of Kerry, Fabius, British Foreign Secretary William Hague, German Foreign Minister Guido Westerwelle and Russian Foreign Minister Sergei Lavrov - along with a Chinese deputy foreign minister - signalled that the six powers were more committed to seeking an elusive pact with Iran than before.
Kerry arrived on Friday from Tel Aviv after what appeared to be a tense meeting with Israeli Prime Minister Benjamin Netanyahu, who rejected any budding compromise with Iran.
Netanyahu warned Kerry and his European counterparts that Iran would be getting "the deal of the century" if they carried out proposals to grant it temporary respite from sanctions in exchange for a partial suspension of, and pledge not to expand, its enrichment of uranium for nuclear fuel.
Israel, which is believed to have the Middle East's only nuclear arsenal and regards its arch-enemy Iran as a mortal threat, has repeatedly mooted military action against Tehran if it does not mothball its entire nuclear programme.
Iran dismisses such demands, citing a sovereign right to a nuclear energy industry, and most diplomats concede that, as Tehran has expanded its nuclear capacity exponentially since 2006, the time for demanding a total shutdown has passed.
But Fabius said the security concerns of Israel and some Arab neighbours of Iran still "have to be taken into account".
Israeli Intelligence Minister Yuval Steinitz may have had the French remarks in mind when he said on Saturday that he "draws encouragement from the fact that there are other partners to Israel's concerns about the agreement shaping up".
Negotiators have limited political room to manoeuvre as there is hardline resistance to any rapprochement both in Tehran - especially among its elite Revolutionary Guards and conservative Shi'ite clerics - and in the US Congress.
Israel's complaints could make it more difficult for President Barack Obama to sell any eventual deal to US lawmakers, who have been far from compliant regarding White House proposals on Syria and numerous domestic issues.
US lawmakers have threatened to slap new sanctions on Iran even as the talks have appeared to progress, despite White House appeals to hold off while negotiations continue.
Eric Cantor, majority leader in the Republican-controlled House of Representatives, said a Geneva deal would fall short if it did not entirely halt Iran's nuclear programme - a demand diplomats say is unrealistic and one that none of the six powers is making.
Criticism also has bubbled up from some leading pro-Israel groups in Washington.
Iran and the powers have been discussing a partial nuclear suspension deal lasting around half a year.
One concession under consideration is the disbursement to Iran in instalments of up to about $50 billion of Iranian funds blocked in foreign accounts for years.
Another step could be temporarily relaxing restrictions on precious metals trade and Washington suspending pressure on countries not to buy Iranian oil.
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