Iran has upheld 10-year sentences on Iranian-American oil executive Siamak Namazi and his 81-year-old father Baqer, a former senior UNICEF official.

An Iranian appeals court maintained the convictions on “collaborating with an enemy state” last week and announced the decision five days later. No written reason was given to the Namazis’ family.

Siamak Namazi was seized in Tehran in October 2015 as he visited relatives. The nature of the charge has never been made public. An executive of Crescent Petroleum, Namazi was a vocal opponent of US sanctions in policy papers written for outlets such as the Woodrow Wilson Center.

Baqer Namazi, an Iranian-Canadian dual national, was detained four months later after he came to Iran to inquire about the status of his son’s case.

Advocates for the Namazis say the health of both men has deteriorated during their detention in Tehran’s Evin Prison. Baqer Namazi, a triple-bypass heart patient, has lost 30 pounds and endured shortness of breath, dizziness, confusion and hearing loss.

The Namazis’ lawyer in Washington, Jared Genser, said Siamak Namazi has spent much of his imprisonment in solitary confinement and has “been interrogated relentlessly, beaten, and tased”.

Iran has regularly seized dual nationals, often on vague claims of espionage for foreign powers. Although five Iranian-Americans were released in January 2016, as Iran’s nuclear deal with the 5+1 Powers was being implemented, other dual nationals were being arrested amid a Revolutionary Guards crackdown.

Those still in Iranian prisons include Anglo-Iranian charity worker Nazanin Zaghari-Ratcliffe, serving five years; Iranian-American Karan Vafadari and his wife Afarin Neyssari, art gallery owners in Tehran; Lebanese-Iranian national and US resident Nazar Zaka, sentenced to 10 years; and Xiyue Wang, a Princeton University Ph.D. candidate in history, also given 10 years.

Zaka’s lawyer Jason Poblete said on Monday that his client’s appeal also had been denied. He passed on a message from Zaka: “If you’re traveling to Iran on business or pleasure, let my case be a lesson and a warning.”