US Secretary of State John Kerry urged Iran on Friday to immediately free an Iranian-American Christian pastor who was sentenced to eight years in jail in January.
Kerry said he was "deeply concerned" about the fate of Saeed Abedini, who has already been held for six months in Iran in the notorious Evin prison.
The top US diplomat said in a statement that he was also "disturbed by reports that Mr Abedini has suffered physical and psychological abuse in prison, and that his condition has become increasingly dire.
"Such mistreatment violates international norms as well as Iran's own laws," Kerry added, saying he was troubled by the lack of due process in the trial, and Iran's refusal to allow Abedini consular access by Swiss authorities.
Abedini, a naturalized US citizen who converted to Christianity, was handed the jail sentence in late January, despite denying "charges of establishing churches to disrupt national security" in Iran and colluding to carry out crimes.
His supporters say he led underground churches in the early 2000s when such activity was largely tolerated during the pro-reform presidency of Mohammad Khatami.
Having resettled in the United States, Abedini was detained on a return trip to Iran in 2009 and was then let go following an agreement not to engage in underground religious activities, according to his family.
"The best outcome for Mr Abedini is that he be immediately released," Kerry added.
Abedini's wife Naghmeh this week released a letter she just received from her husband alleging that he has beaten during interrogation, and that he has suffered from "intense pain."
The United States and Iran broke off diplomatic ties in the late 1970s and Switzerland now handles consular matters for Washington with the Iranian authorities.