The family of an Iranian-American businessman and his 80-year-old father say the two men have been sentenced to 10 years in prison by an Iranian court that convicted them of collaborating with the United States.
The detentions of Siamak and his father, Baquer Namazi, have drawn protests from the United States.
Siamak Namazi has been in Iranian custody since being detained on his arrival in Tehran in October 2015.
Baquer Namazi, a retired UNICEF official who also holds dual Iranian-American citizenship, was detained in February after traveling to Iran to seek his son's release.
The Mizan news agency, which is controlled by Iran’s judiciary, reported on October 18 that the two were jailed for "collaborating with the hostile American government.”
Siamak's brother, Babak, said in a statement obtained by RFE/RL that the 10-year sentence issued on October 18 amounted to a life sentence for their father.
“It is beyond comprehension that the court and those holding our loved ones have criminalized the humanitarian efforts of my father and brother,” he said.
Friends have described Siamak Namazi as a businessman who avoided politics except for a time when he lobbied Washington to end a U.S. embargo on selling medicines to Iran.
Earlier this week, Mizan posted a minute-long video showing a montage of clips including Siamak in a conference room, his arms raised at his sides, and his U.S. passport.
UNICEF, where Baquer Namazi had worked as a representative to Somalia, Kenya and Egypt, took note of the sentence with "sadness and personal concern," said Stephane Dujarric, spokesman for United Nations Secretary-General Ban Ki-moon.
Dujarric added that Namazi "deserves a peaceful retirement and UNICEF appeals for his release on humanitarian grounds."
The United States also expressed "deep concern" about the two men being "unjustly detained," said Mark Toner, a spokesman for the U.S. State Department.
"We join recent calls by international organizations and UN human rights experts for the immediate release of all U.S. citizens unjustly detained in Iran, including Siamak and Baquer Namazi, so that they can return to their families," Toner said.
In recent months, several Iranians who hold dual nationalities, including American, have been detained in Iran.
It is unclear exactly what has prompted the detentions, although some observers say it is part of a behind-the-scenes struggle between allies of the reform-minded President Hassan Rohani and hard-line factions such as the powerful Islamic Revolutionary Guard Corps (IRGC).
The detentions also come as channels of communication slowly open between Tehran and the West following last year's landmark agreement that lifted punishing international sanctions in exchange for restrictions on Iran's disputed nuclear program.