France said on Saturday there was no certainty nuclear talks under way with Iran in Geneva would succeed because of major stumbling blocks over an initial proposed text on a deal, and the importance of Israel's security concerns.

"As I speak to you, I cannot say there is any certainty that we can conclude" the talks, French Foreign Minister Laurent Fabius said on France Inter radio, noting that France could not accept a "sucker's deal".

Top diplomats from Iran and six world powers are in a third day of negotiations over Iran's nuclear program to try to end a decade-old standoff. The West suspects that Iran is enriching uranium to develop nuclear weapons capability. Tehran denies this.

"The security concerns of Israel and all the countries of the region have to be taken into account," 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 it of being more intransigent in talks than the United States.

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.

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.

The talks have deeply upset some of Iran's traditional enemies in the region such as Israel, whose Prime Minister Binyamin Netanyahu said the Jewish state "utterly rejects" a deal.

Meanwhile, Western diplomats say they are disappointed at the progress of nuclear negotiations in Geneva, and expect discussions to continue at least until next week, Los Angeles Times reported on Friday.

Unnamed diplomats from Washington did not tell the Times why the talks had come to a halt, but the paper speculated that the disagreement could be over Iran's construction of a plutonium reactor, a direct path to a nuclear bomb.

Part of the proposed agreement between Iran and the international community is that they would agree not to activate the reactor while an interim deal is in place.

Iranian Deputy Foreign Minister and Chief Negotiator Abbas Araghchi was also skeptical about the success of the current talks, saying "negotiations have reached [a] critical, very sensitive situation, and it needs decisions at higher levels."

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.

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