The fate of Iran's heavy-water reactor has become a sticking point in high-level nuclear negotiations in Geneva, according to the French foreign minister, Laurent Fabius.
The Iranian delegation is believed to have presented western powers with a draft text of an agreement on Friday, which is now the focus of the negotiation. But Fabius told France Inter radio on Saturday that Paris would not accept a "sucker's deal". He said: "As I speak to you, I cannot say there is any certainty that we can conclude."
There were reports that France was taking a harder line than other western states, potentially complicating an already tangled issue. But one western source familiar with the negotiations played down talk of divisions, saying Paris was only voicing caution about accepting the Iranian text as a fait accompli.
That caution was shared by other nations, the source said, but there was also hope that the remaining obstacles could be overcome.
The talks reached a critical moment on Saturday, with the convergence in Switzerland of six foreign ministers in an attempt to break the decade-long deadlock over the Iranian nuclear programme. Russia's Sergei Lavrov, arrived to join the US secretary of state, John Kerry. The Chinese foreign minister, Wang Yi, was expected to arrive on Saturday evening.
The European Union's foreign policy chief, Catherine Ashton, who is co-ordinating the talks, hosted a five-hour meeting between Kerry and the Iranian foreign minister, Mohammad Javad Zarif, on Friday night, and the three met again at lunchtime on Saturday.
Ali Vaez, an analyst for the International Crisis Group, observed: "The amount of time Iranian and American top diplomats have spent talking to each other in the last 24 hours is more than the preceding 34 years."
While western officials warned that a deal might not be concluded in the current round, they also argued that to put off an agreement could result in a fatal loss of momentum.
"These negotiations have made good progress and continue to make good progress," Britain's foreign secretary, William Hague, said. "But there are still important issues to resolve. We are going to have give a lot of time and attention to those issues in the course of today and there is no fixed time for us to reach a conclusion.
"We are very conscious that some real momentum has built up in these negotiations. There is now a real concentration on these negotiations. So we have to seize the moment to reach a deal that has eluded the world for a very long time."
A western official added: "The key word is momentum. There is forward movement. We are getting somewhere." But Hague said that because Iran's nuclear programme was so extensive, any deal would have to be very detailed to assure international confidence that it was for purely peaceful purposes.
Fabius said one of the key issues was the western call for construction work on the heavy-water reactor at Arak to stop as part of an interim deal aimed at buying time for negotiations on a more comprehensive long-term deal.
After many delays, the Arak reactor is due to be commissioned next year. Iran says its purpose is to produce nuclear isotopes which are useful for medical and agricultural purposes. But when operating it would produce plutonium as a byproduct in its spent fuel and that plutonium would represent a serious proliferation risk.
Fabius said: "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."
Iran's senior nuclear negotiator and deputy foreign minister, Abbas Araqchi, said: "There is still a distance between Iran's stance and that of the P5+1 [the six-nation negotiating group," according to the Isna news agency. "The talks will end tonight." If anything is left to be discussed, he said will instead be considered in the next round of negotiations rather than on Sunday. He did not elaborate on the differences between the two sides.
Daryl Kimball, the executive director of the Arms Control Association, in Washington, argued that the heavy-water plant at Arak should not be an obstacle to achieving a stop-gap deal to defuse tensions.
Kimball said construction work "is more than a year from being completed; it would have to be fully operational for a year to produce spent fuel that could be used to extract plutonium. Iran does not have a reprocessing plant for plutonium separation and Arak would be under IAEA safeguards the whole time.
"Arak represents a long-term proliferation risk not a near-term risk and it can be addressed in the final phase of negotiations. France and the other … powers would be making a mistake if they hold up an interim deal that addresses more urgent proliferation risks over the final arrangements regarding Arak."