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A 75-year-old Austrian citizen of Iranian origin, on charges of espionage, has been given a five-day release from , his daughter told AFP on Monday.

"He was freed for medical reasons after three and a half years' detention," said Fanak Mani, the daughter of Massud Mossaheb.

Mossaheb is being held at , near the capital Tehran. A photo posted on the Free Massud Twitter account shows him smiling, supporting himself on a crutch, with his wife by his side.

Mossaheb was the general secretary of the Austro-Iranian Society, an organisation set up to encourage exchanges between the two countries.

He was arrested in Tehran in January 2019 while he was accompanying a visiting group of Austrian scientists. Accused of spying , he was sentenced to 10 years in jail.

His family has raised concerns about the state of his health on several occasions.

More than 10 dual nationality Iranians from western countries are currently being held by the authorities in the country.

Rights groups such as Amnesty International have condemned their detention as "hostage-taking" for political ends, as a means to put pressure on western powers to win concessions.

Iran, which does not recognise double nationality, says that all the people held have been detained following rulings by the courts.

Those detained include four , another Austro-Iranian, and others with dual nationality in Sweden, , Britain and the United States.