Diplomats said ahead of today's meeting that progress is being made but significant gaps remain on a main document and technical annexes ahead of an end-of-June deadline.
Iran's team is led by Deputy Foreign Minister Abbas Araghchi. EU official Helga Schmidt is heading the other side.
Senior officials from the US, Russia, China, Britain, France and Germany may join in later.
The deal aims for strict limits on Iranian nuclear capabilities that could be used to make weapons. Tehran denies any interest in such arms but is negotiating for a lifting of sanctions.