This photograph taken in Tournai on January 12, 2023 shows a giant banner with a portrait of Belgian aid worker Olivier Vandecasteele who was arrested by the Iranian authorities on February 24, 2022. AFP
"We believe Mr Vandecasteele has been arbitrarily deprived of his liberty and is a victim of enforced disappearance for periods of detention," a group of eight independent experts said in a statement.
The experts, including the UN special rapporteur on the rights situation in Iran, and members of the working group on enforced or involuntary disappearances, warned that the aid worker's "right to a fair trial before an independent and impartial tribunal has been violated".
"These are flagrant violations of Iran's obligations under international law," they said.
Vandecasteele, 41, was last week handed multiple sentences totalling 40 years on a range of charges, including "espionage", but with the sentences to run concurrently he will serve 12 and a half years in jail, the judiciary's Mizan Online website reported.
He was also sentenced to 74 lashes.
Iran arrested Vandecasteele in February 2022, and he has since been held in conditions that Belgium's government has described as "inhumane".
Supporters of Vandecasteele and rights groups contend he is being held as part of Iran's "hostage diplomacy", an effort by Tehran to get Belgium to release an Iranian diplomat incarcerated for terrorism.
In Tuesday's statement, the experts, who are appointed by the United Nations Human Rights Council but who do not speak on behalf of the world body, maintained that Vandecasteele had suffered ill-treatment in detention, warning that his health condition was "critical".
They also said he had been denied access to a lawyer, was not permitted to communicate regularly with his family and also only had limited access to Belgian consular services.
Last November, 10 months after his arrest, he launched a two-week hunger strike.
Citing reports, the experts said the Belgian had lost 15 kilograms (33 pounds) and "suffers from serious health problems".
"We are gravely concerned over the mental and physical health of Mr. Vandecasteele and call on the Iranian authorities to release him and guarantee his physical and psychological wellbeing," they said.
As of last October, at least 21 foreign and dual nationals were detained in Iran, primarily on espionage charges, they said.
Among them was British-Iranian dual national Alireza Akbari, whose execution last Saturday, sparked international outrage.
The experts said they were "appalled by Akbari's execution", pointing out that he was reportedly tortured and "forced to make false confessions."
They also called on Tehran to "end the institutionalised practice of hostage-taking and to release the many foreign and dual nationals who remain arbitrarily detained."