Six world powers and Iran are meeting in an attempt to find common ground over concerns that Tehran's nuclear programme could be used to make weapons.
Iran insists it has no such ambitions, but the international community fears it could use its uranium enrichment program not only to make reactor fuel but also the fissile core of nuclear weapons.
After years of futile meetings, both sides have expressed optimism that enough progress could be achieved this time for a second round of talks.
The six - the United States, Russia, China, Britain, France and Germany - hope Iran will commit to at least discussing their concerns over its enrichment program today. That is something Tehran has refused to do during the most recent meetings.