Iranian Foreign Minister Mohammad Javad Zarif, US Secretary of State John Kerry and European Union's (EU) nuclear coordinator Catherine Ashton started their tripartite talks here Sunday.
They are expected to reach a comprehensive deal over Iran's nuclear programme after earlier talks, spread over many months, failed to reach a final agreement, IRNA news reported.
The talks will focus on disputes between Iran and the P5+1 group - Britain, China, France, Russia, the US plus Germany - concerning enrichment, lifting of sanctions and the duration of the agreement.
A second round of negotiations is scheduled to be held later in the day.
A major round of talks is also scheduled to be held again in Vienna Nov 18 before the Nov 24 deadline.