In the trilateral meeting that started on Thursday morning, Iranian negotiators Seyed Abbas Araqchi and Majid Takht Ravanchi held talks with US Undersecretary of State Wendy Sherman and the European Union's deputy secretary general for the external action service, Helga Maria Schmid.
The EU coordinates diplomacy with Iran on behalf of the Group 5+1 (Russia, China, the US, Britain, France and Germany).
Political directors general of the six world powers are also scheduled to join the talks on Friday.
In this round, negotiators are trying to reach a comprehensive text of a final deal that covers the points about which the two sides disagree.
On the first day of this round, expected to run until Friday, the Iranian deputy foreign ministers held a meeting with the EU's Schmid in a bid to resume drafting the text of a final deal on Tehran's peaceful nuclear program ahead of a self-imposed deadline of June 30.
Iran and the G5+1 (also known as the P5+1 or E3+3) are in talks to hammer out a lasting accord that would end more than a decade of impasse over Tehran's peaceful nuclear program.
On April 2, the two sides reached a framework nuclear agreement in Lausanne, Switzerland, with both sides committed to push for a final deal until the end of June.