The negotiations entered their 16th day on Sunday with a meeting between Iranian and American diplomats.
Seyed Abbas Araqchi and Majid Takht Ravanchi, deputies to Iranian foreign minister, held a meeting with US Under Secretary of State Wendy Sherman to finalize the text of a long-awaited nuclear accord.
Iran's Foreign Minister Mohammad Javad Zarif and US Secretary of State John Kerry are also going to meet later in the day.
The two men have met nearly every day since Kerry arrived in Vienna more than two weeks ago.
"Still have difficult issues to resolve," Kerry tweeted on Saturday after meeting Zarif.
On Saturday, France's foreign minister said the time had come to make a decision.
"Now that everything is on the table, the moment has come to decide," Laurent Fabius said in a statement sent to Reuters after speaking to Zarif, Kerry, and British Foreign Secretary Philip Hammond.
Fabius and Hammond returned to Vienna on Saturday and a US official said that Kerry had spoken by telephone with Russian Foreign Minister Sergei Lavrov, who was not in Vienna.
Iran and the six powers have given themselves until Monday to reach a deal. An initial self-imposed deadline of June 30 for the final accord has been extended three times in the past two weeks.