Two men are staging a hunger strike to demand the release of prisoners being held in Iran, on the sidelines of international negotiations in Vienna to halt Tehran's nuclear ambitions.
The two activists have also experienced being jailed, they told dpa in Vienna.
As a young diplomat from the US, Barry Rosen was detained during the Iranian hostage crisis from 1979 to 1981, along with dozens of other embassy staff, he said on Saturday. He has been on a hunger strike since Wednesday.
Nizar Zakka, who began his hunger strike at the same time, was arrested in 2015 for alleged espionage. The US-Lebanese citizen was jailed for four years.
Currently, more than a dozen people from Western countries are being held in Iran, for alleged political offenses or espionage.
"I want Iran to release all these hostages immediately," Rosen told dpa.
He is calling on the US, Germany, France and Britain to put Tehran under greater pressure to bring about their citizens' release.
It is also important to send a signal to the prisoners that they have not been forgotten, Zakka said.
"This crisis has our full attention," senior US diplomat Robert Malley said in a tweet after meeting with Rosen in Vienna. However, he called on Rosen to end the hunger strike to protect his health.
Rosen has taken up residence at the Palais Coburg hotel where the talks are taking place. Zakka is staying at the Hotel Imperial, where the US delegation is based.
Malley and his colleagues have spent months trying to find a way to get Iran and the US signed back on to a 2015 deal - the Joint Comprehensive Plan of Action - under which the US and five other countries agreed to lift sanctions on Iran in exchange for Iran halting its pursuit of nuclear weapons.
But the US never lifted non-nuclear sanctions and then, former US president Donald Trump pulled America out of the deal, prompting Iran to start violating some of its terms.
US Secretary of State Antony Blinken spoke of only "modest progress" in the nuclear negotiations during a visit to Berlin on Thursday.
Rosen and Zakka say the West should not agree to a nuclear solution unless they also receive guarantees that prisoners will be released.
Rosen said he had been tortured and that those being held in Tehran were also suffering in the same way. "The psychological impact of that imprisonment has never left me," Rosen said.
