The main aims of the negotiations are to get the US to return to the deal and lift sanctions, and for Iran to resume full compliance with the 2015 accord.
Chief negotiators from Iran and Europe have returned home for consultations as talks to revive the 2015 nuclear deal reached a critical stage.
"The negotiators will return to Vienna in two days" but expert-level discussions at the eighth round of talks would continue on Saturday and Sunday, IRNA news agency said on Saturday.
"We are now at a stage of the negotiations where we are discussing difficult issues and how we can translate the subjects that we agreed upon in principle into words and enter them into a document," IRNA quoted an anonymous source as saying.
The main aims of the negotiations are to get the United States to return to the deal and lift sanctions, and for Iran to resume full compliance with the accord.
Tehran is seeking verification of the sanctions easing, as well as guarantees that Washington will not withdraw from the deal again. US president Donald Trump unilaterally withdrew from the agreement in 2018.
"Regarding the three subjects (lifting of sanctions, nuclear commitments and implementation, sequencing and verification), there are still open issues and some of them are tough," the source said.
The return of the negotiators to their capitals came as EU foreign policy chief Josep Borrell said on Friday that a deal with Iran remained "possible", and that the talks were advancing in a "better atmosphere" than before Christmas.
Iran's foreign ministry spokesperson Saeed Khatibzadehsaid on Monday said the efforts by "all parties" to revive the nuclear agreement had resulted in "good progress".
French Foreign Minister Jean-Yves Le Drian recently noted progress in the talks, but said it was "too slow".
US Secretary of State Antony Blinken said on Thursday that only "a few weeks" were left to save the 2015 deal, and that Washington would consider "other options" if the negotiations fail.
The talks between Tehran and world powers resumed in late November after they were suspended for around five months as Iran elected a new, ultraconservative government.