U.N. human rights experts announced on Thursday that Swedish-Iranian scientist Ahmadreza Djalali, sentenced to death in Iran on espionage charges, is in critical condition and near death after months of prolonged solitary confinement.

“Djalali’s situation is truly horrific,” the experts said. In an appeal calling on Tehran to free him, they said he had been held in solitary confinement for more than 100 days, with prison officials shining bright lights in his cell round the clock to prevent him from sleeping.

“Medical issues have prevented him from eating properly, resulting in dramatic weight loss,” said the experts, who include the U.N. special rapporteurs on the situation in Tehran, arbitrary executions, arbitrary arrests and torture.

“His situation is so difficult that he reportedly has trouble speaking. We are shocked and distressed by the cruel mistreatment of Mr. Djalali.”

A call to the Iranian diplomatic mission in Geneva seeking comment was not immediately returned.

Djalali, a medical doctor and lecturer at the Karolinska Institute in the Swedish capital Stockholm, was detained in Iran in 2016 and later convicted of espionage, accused of providing information to Israel to help it assassinate nuclear scientists. Iran’s Supreme Court in 2017 upheld the death sentence.

The U.N. experts noted that Djalali’s conviction and sentence were based on a confession extracted under torture and after an unfair trial. His treatment was emblematic of Tehran’s systematic use of solitary confinement “to punish and pressurize detainees, including to make forced confessions,” they added.

Rights activists have accused Tehran of jailing a number of dual nationals in an attempt to win concessions from other countries. Iran has regularly dismissed the accusations, claiming its judicial system is fair.