The Iranian delegation is led by Foreign Minister Mohammad Javad Zarif and the Americans are headed by Secretary of State John Kerry.
Iranian Deputy FMs Seyed Abbas Araghchi and Majid Takht-Ravanchi are also present in the talks alongside their American counterpart Wendy Sherman.
The French Foreign Minister Laurent Fabius who left Vienna for Paris on Friday is now back here to join the talks.
Back in Vienna on Friday, German Foreign Minister Frank-Walter Steinmeier had a meeting with Kerry on Saturday morning.