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 yesterday 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 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 this weekend.

"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 said.

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% 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 as 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 co-ordinating negotiations with Iran on behalf of the six nations, said she continued "intense" talks and contacts yesterday with the parties involved.

Ashton, US Secretary of State John Kerry and Zarif are 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.

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% in the past two years.

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.