The US special envoy for Iran on Sunday condemned Tehran's continued jailing of human rights lawyer Nasrin Sotoudeh, saying "she should not have spent a single day in prison".
Robert Malley tweeted a message of solidarity on the third anniversary of Ms Sotoudeh's imprisonment.
"It's been three yrs since human rights lawyer Nasrin Sotoudeh was sentenced to 38.5 yrs in prison and 148 lashes for defending women's rights in Iran. From prison she [continues] to advocate for the humane treatment of political prisoners. She should not have spent a single day in prison," he wrote.
Amnesty International has described the two trials held against Ms Sotoudeh "grossly unfair".
While she was released for a month last year, following a 46-day hunger strike that ended in September, Ms Sotoudeh was ordered back to prison in December.
Mr Malley's tweet came as he led the US delegation at the sixth round of indirect negotiations with Iran, underway in Vienna. Representatives from China, Germany, France, Russia, and Britain have been holding direct meetings with the Iranians since Saturday, while Washington is engaged in indirect talks.
The goal is to return to the nuclear deal signed in 2015 between Iran and the world powers. But several hurdles such as sanctions relief and Iran's breaches of the deal by expanding its programme have prevented a breakthrough.
Russia's representative Mikhail Ulyanov said an agreement was "within reach", but not likely before Iran's presidential elections on Friday.
"It is almost [certain] that we will not be able to clean up the draft final document by the day of the Presidential elections in Iran. Nevertheless an agreement is within reach," he said.