Friday's talks follow a previous round in February in the Kazakh city of Almaty that ended with both sides speaking of some movement in bridging significant differences.
The United States, Russia, China, Britain, France and Germany hope the talks will result in at least some movement in a decade of efforts to reduce Iran's bomb-making capacities by curbing its uranium enrichment program.
Iran wants an end to punishing sanctions imposed to force it to end uranium enrichment, a process that can generate both nuclear energy and the core of nuclear weapons. It denies any interest in atomic arms.
Another round is scheduled for Saturday.