US Secretary of State John Kerry speaks to the press upon his arrival in Geneva

France warned of serious stumbling blocks to a long-sought deal over Iran's nuclear programme as foreign ministers from the Islamic state and six world powers extended negotiations into a third day on Saturday to end a decade-old standoff.

As the talks stretched on, French Foreign Minister Laurent Fabius said there was no certainty they would succeed in nailing down an interim deal that would begin to defuse fears of a covert Iranian advance towards nuclear arms capability.

"As I speak to you, I cannot say there is any certainty that we can conclude," Fabius said on France Inter radio, stressing that France could not accept a "sucker's deal". British Foreign Secretary William Hague said the talks have achieved "very good progress" but important issues remained unresolved and he did not know whether a deal could be clinched by the end of the day.

"We are very conscious of the fact that real momentum has built up in these negotiations and there is now real concentration on these negotiations and so we have to do everything we can to seize the moment," he told reporters. Among the sticking points, Fabius said, was a call for Iran to halt operations at its Arak research reactor - a potential producer of bomb-grade plutonium - while the negotiating process goes on, as well as questions about Iran's stock of uranium enriched to 20 percent of fissile purity.

Both issues are at the heart of Western concerns that the Islamic Republic is stockpiling enriched uranium not for civilian nuclear power stations, as Tehran says, but rather potential fuel for atomic bombs. "We are for an agreement, that's clear. But the agreement has got to be serious and credible.

The initial text made progress but not enough," Fabius said. France has traditionally taken a tougher line on Iran than most other world powers and Iranian Foreign Minister Mohammad Javad Zarif has accused Paris of being more intransigent in talks than Tehran's longtime main adversary, the United States.

The spokesman for European Union foreign policy chief Catherine Ashton, who is coordinating negotiations with Iran on behalf of the six nations, said she continued "intense" talks and contacts on Saturday morning with the parties involved. Ashton, US Secretary of State John Kerry and Zarif were later expected to resume trilateral talks after a five-hour session on Friday evening.

They were searching for an agreement that would cap Iran's nuclear capacity and make it more transparent in exchange for phased, initially limited, relief from international sanctions that are choking its oil-based economy. The goal is to take a first step towards resolving a protracted dispute that could otherwise plunge the volatile and oil-rich region into a new conflict. "We're working hard," Kerry told reporters as he arrived at his hotel shortly before midnight (2300 GMT) on Friday.

A senior State Department official said: "Over the course of the evening, we continued to make progress as we worked to the narrow the gaps. There is more work to do." Iranian Deputy Foreign Minister Abbas Araqchi said: "It was productive but still we have lots of work to do."

Iran and the powers have remained quiet about the nitty gritty of negotiations, eschewing the mutual accusations of bad faith typical of past, sporadic meetings over the past decade. Diplomats involved in the talks say the change shows how committed to a substantive agreement both sides have become. Kerry cautioned after arriving on Friday that "there are some important gaps that have to be closed."

Iran spelled out one major bone of contention. A member of its negotiating team, Majid Takt-Ravanchi, told Mehr news agency on Friday that oil and banking sanctions imposed on Tehran should be eased during the first phase of any deal. The powers have offered Iran access to up to $50 billion in Iranian funds frozen abroad for many years but ruled out any broad dilution of sanctions in the early going of an agreement.

Diplomats said a breakthrough remained uncertain and would in any case mark only the first step in a long, complex process towards a permanent resolution of concerns about the nature of Iran's nuclear quest. But they said the arrival of Kerry, Fabius, Hague and German Foreign Minister Guido Westerwelle signalled that the five permanent U.N. Security Council members and Germany may be closer to an elusive pact with Iran than ever before. Russian Foreign Minister Sergei Lavrov was expected to join them later on Saturday.

Kerry arrived on Friday from Tel Aviv after what appeared to be a tense meeting with Prime Minister Benjamin Netanyahu, who rejected any 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 Tehran limited, temporary relief 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 repeated mooted possible military action against Tehran if it did not mothball its entire nuclear programme. "The security concerns of Israel and all the countries of the region have to be taken into account," Fabius said.

Still, the fact that a deal may finally be feasible after a decade of feuding rather than genuine negotiations between Iran and the West highlighted a striking shift in the tone of Tehran's foreign policy since the election in June of Hassan Rouhani, a pragmatic former nuclear negotiator, as president.

Iran, which harbours some of the world's largest oil and gas reserves, wants the six powers to lift increasingly tough restrictions that have slashed its daily crude sales revenue by 60 percent in the past two years. Iran and the powers are discussing a partial nuclear suspension deal covering around half a year.

If a preliminary deal is nailed down, it would only be the first stage in a process involving many rounds of intricate negotiations in the next few months aimed at securing a permanent agreement. One idea under consideration is the disbursement in instalments of up to about $50 billion of Iranian funds blocked in foreign accounts for decades.

Another step could be temporarily relaxing restrictions on precious metals trade. A further step could be Washington suspending pressure on countries not to buy Iranian oil. Diplomats say that such a move by Washington could be immediate and easily reversible if Iran failed to meet its obligations under a deal.

Negotiators have limited political room to manoeuvre as there is hardline resistance to any rapprochement in Tehran - especially its elite Revolutionary Guards - and in the US Congress and either could make it hard to implement any deal.

The Obama administration has said world powers will consider some sanctions relief, while leaving the complex web of US, EU and U.N. restrictions in place, if Iran takes verifiable steps to rein in its nuclear programme. Lending urgency to the need for a breakthrough was a threat by the US Congress to pursue tough new sanctions on Iran.

Obama has been urging Congress to hold off on more punitive steps to isolate Iran, demanded by Israel, to avoid undermining the delicate diplomatic opening with the country. But many US lawmakers, including several of Obama's fellow Democrats, believe tough sanctions forced Iran to the negotiating table in the first place and that more are needed to discourage it from diverting enrichment toward bomb-making.

(Additional reporting by Louis Charbonneau and Stephanie Nebehay Dahl in Geneva, John Irish and Leigh Thomas in Paris, editing by Mark Heinrich)