Alexei Navalny to be medically evacuated and treated abroad, saying they believed his life was at risk.
Navalny has been kept in harsh conditions in a high-security penal colony and "denied access to adequate medical care", conditions that may amount to torture, they said in a statement.
"We urge the Russian authorities to ensure Mr. Navalny has access to his own doctors and to allow him to be evacuated for urgent medical treatment abroad, as they did in August 2020," said the UN experts.
The Kremlin critic, 44, began a hunger strike three weeks ago. He is serving a 2-1/2-year sentence on old embezzlement charges that he says were trumped up.
Navalny returned to Russia in January after treatment in Germany for what German authorities say was poisoning in Russia with a banned nerve agent. The Kremlin denies any blame.
The UN. experts voiced alarm at his deteriorating health, saying: "We believe Mr. Navalny's life is in serious danger."
"We are deeply troubled that Mr. Navalny is being kept in conditions that could amount to torture or cruel, inhuman or degrading treatment or punishment in a facility that reportedly does not meet international standards," they added.
They said that Navalny's current imprisonment and past attacks on him, including with Novichok, are "all part of a deliberate pattern of retaliation against him for his criticism of the Russian government and a gross violation of his human rights".