Diplomats grappled to secure a landmark deal at talks on Iran's disputed nuclear programme on Saturday after Tehran raised the prospect of negotiations being put off.
In a rush of late-night diplomacy, Iranian Foreign Minister Mohammad Javad Zarif had joined foreign ministers from six world powers for a session expected to be the last before talks break up.
At stake was a proposed deal that could see Iran freeze parts its nuclear programme in exchange for the easing of some of the sanctions that are crippling its economy, as a first step ahead of more talks.
US and European officials said Zarif had joined a meeting attended by EU diplomatic chief Catherine Ashton and ministers from the P5+1 group that includes Britain, China, France, Germany, Russia and the United States.
Iranian President Hassan Rouhani had earlier urged world powers not to miss the chance for a deal.
"I hope that the P5+1 group make the most out of this exceptional opportunity that the Iranian nation has offered to the international community, so that we can reach a positive result within a reasonable timeframe," he was quoted as saying by the official IRNA news agency.
The US, British, French, German and Russian foreign ministers had rushed to Geneva to join the talks amid hopes of a breakthrough toward ending the decade-old standoff on Iran's nuclear efforts.
Officials including US Secretary of State John Kerry, who cut short a Middle East tour to attend the talks, have said progress was being made but cautioned that serious obstacles remained.
After several meetings with fellow diplomats, Zarif said earlier that there were still "disagreements" on some questions and pointed to divisions among world powers.
"There are differences of opinion within the P5+1 group," he was quoted as saying by Iranian news agency ISNA, adding that negotiations would not continue into Sunday if a deal was not reached.
"If we do not reach an agreement tonight, the talks will be resumed in the next seven or 10 days," he said.
France in particular had raised concerns about the purported deal, with Foreign Minister Laurent Fabius saying there was "no certainty" an agreement would be reached.
Fabius cited calls for Iran to halt operations at its Arak nuclear reactor and questions about enriched uranium stockpiles as particular points of contention.
"If these questions are not settled, it will not be possible" to reach a deal, Fabius told France Inter radio, saying an agreement also needed "to take fully into account Israel's security concerns".
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Using unusually harsh language, a Western diplomat said France was complicating the talks.
"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 AFP.
Pressure was high for the diplomats to finally reach a deal after a decade of negotiations between Iran and the P5+1 group.
Officials had a series of meetings on Saturday, with Russian Foreign Minister Sergei Lavrov and Chinese Vice Foreign Minister Li Baodong also arriving to join the talks.
The talks began on Thursday and on Friday went on until almost midnight.
Reports say the proposed deal could see Iran stop enriching uranium to 20 percent, which is just a few technical steps from weapons-grade, reduce existing stockpiles and agree not to activate its plutonium reactor at Arak.
Global powers would in exchange take limited and "reversible" measures to ease sanctions, such as unfreezing some Iranian funds in foreign accounts.
Negotiators would then have time to work out a more comprehensive deal that Iran has said it hopes could be in place within a year.
The possible agreement already came under fire from Tehran's arch-foe Israel, widely thought to be the Middle East's sole if undeclared nuclear armed power, which has opposed any move to ease sanctions.
"This is a very bad deal. Israel utterly rejects it," said Israeli Prime Minister Benjamin Netanyahu.
The world powers in the talks suspect Iran's programme is aimed at developing nuclear weapons, despite Tehran's repeated denials.
Iran also expects to sign an accord with the chief of the UN nuclear watchdog on Monday that will include "concrete actions", Tehran's ambassador to the agency, Reza Najafi, told state television.
The two tracks of talks -- with the IAEA and P5+1 -- over Iran's atomic activities were given new momentum by the June election of Rouhani, seen as a relative moderate.
Iran is anxious for relief from crippling US and European Union economic sanctions that have cut oil revenues by more than half, caused the value of its currency to plunge and pushed inflation above 40 percent.