Iranian authorities have allowed a French-Iranian academic held in the country for the last three years to take brief leave from prison, her supporters said on Tuesday.

Fariba Adelkhah has been given a five-day furlough, which can be extended, from Tehran's Evin prison where she is being held in a case that has raised tensions between Tehran and Paris, a statement from her support group said.

Activists say that at least 20 foreign and dual nationals are being held by Tehran on baseless charges, in a deliberate policy of hostage diplomacy aimed at extracting concessions from the West.

Her temporary release comes at a crucial time in the negotiations between world powers and Iran over the Iranian nuclear programme, with Tehran studying a final proposal from the EU aimed at salvaging a 2015 deal.

"Fariba has been granted a five-day period of leave, which is potentially renewable, and of course under judicial supervision," her supporters said in a statement.

"We are delighted... but can only emphasise that this unacceptable deprivation of liberty is within the framework of a public policy of hostage taking by Iran," the statement added.

It is relatively common for prisoners in Iran to be allowed brief leave for time at home with families before returning to jail.

A specialist in Shiite Islam and a research director at Sciences Po university in Paris, Adelkhah was arrested in June 2019 along with her French colleague and partner Roland Marchal.

Adelkhah was sentenced in May 2020 to five years in prison for conspiring against national security, accusations her supporters have always denounced as absurd.

Marchal was released in March 2020 and Adelkhah was allowed home in Tehran in October 2020 with an electronic bracelet. But she was then sent back to prison in January 2022.

Iran last month allowed German-Iranian woman Nahid Taghavi, who was arrested in October 2020, a medical furlough to get treatment for back and neck problems.

Three other French nationals are also being held by Iran.

Benjamin Briere, who according to his family is simply a tourist, was arrested in May 2020 after taking pictures in a national park with a recreational drone and sentenced to eight years in prison on spying charges.

Meanwhile, French teachers' union official Cecile Kohler and her partner Jacques Paris were arrested in early May on security-related charges, Tehran has said.

Iran insists the foreigners are given fair trials but their families argue they are being held as pawns in a political game.