Negotiators are meeting in Vienna in attempts to reach a deal that limits Iran's nuclear program in exchange for sanctions relief.
U.S. Undersecretary of State Wendy Sherman, Iranian Deputy Foreign Minister Abbas Araghchi, and EU official Helga Schmid began talks on April 22 in a Vienna hotel.
The talks are fine-tuning a framework agreement reached earlier this month.
Iran denies any interest in nuclear arms but has agreed to reduce the capacity of programs that could be used to make such weapons. It wants an end to sanctions in exchange.
Complicated details on timing and monitoring remain to be ironed out by the June 30 deadline.
U.S. Secretary of State John Kerry and the foreign ministers of Iran and and five other countries are expected to join in the talks at a later stage.