"We are deeply concerned of reports that Iranian-American scholar Kian Tajbakhsh was recently charged by an Iranian court without the benefit of a lawyer," State Department deputy spokesman Robert Wood said.
"Given that the charges facing Mr Tajbakhsh are without foundation, we call on Iran's leadership to release Mr Tajbakhsh without delay," he added, reading a statement at the daily news briefing.
Iran on Saturday put on trial around 100 people, including Tajbakhsh, top reformists and aides of opposition leaders, on charges of rioting after the disputed re-election in June of Mahmoud Ahmadinejad as president.
The trial in a revolutionary court came as Ahmadinejad, due to be sworn in on August 5 for a new four-year term, sought to ease political tensions by denying a rift with all-powerful supreme leader Ayatollah Ali Khamenei.
"As an independent academic, Mr Tajbakhsh has always sought political neutrality," Wood said.
"The right to due process in Iran, which includes the right to legal representation, is not only addressed in the universal declaration of human rights to which Iran is a signatory," it is also contained in its own constitution, he said.
