The source quoted a senior Iranian official on Sunday, rejecting earlier reports about the probability of a provisional agreement on a historic nuclear deal.
The two sides are working hard but a deal tonight is simply impossible, the official said, adding that the document will be roughly 100 pages.
The remaining differences will be also discussed by the foreign ministers of the negotiating sides, the source said.
Meanwhile, a senior US State Department official cast doubt on the “timing of anything” and said, “We have never speculated about the timing of anything during these negotiations and we are certainly not going to start now, especially given the fact that major issues remain to be resolved in these talks.”
US Secretary of State John Kerry said on Sunday that a “few tough things” remain in the way, adding, “We’re getting to some real decisions.”
Kerry said he was “hopeful” after a “very good meeting” with Iranian Foreign Minister Mohammad Javad Zarif on Saturday.
With a new Monday deadline for a deal looming, the Iranian foreign minister and US secretary of state held two rounds of talks on Saturday. Zarif and Kerry also held another meeting in the presence of the European Union foreign policy chief, Frederica Mogherini.
Iran and the five permanent members of the UN Security Council – the United States, Britain, France, Russia and China – plus Germany have been holding intensive talks over the past 16 days in the Austrian capital of Vienna to iron out the remaining differences and nail down a landmark deal on Tehran’s nuclear program.
French Foreign Minister Laurent Fabius also said on Sunday that the marathon nuclear talks between Iran and the P5+1 countries are now in “the final phase.”
“I hope we are finally entering the final phase of these marathon negotiations. I believe it,” Fabius told reporters in Vienna.