Russia's Nobel-Prize-winning Memorial human rights group on Monday expressed concern over the condition of its jailed co-founder, Oleg Orlov, imprisoned for criticising Moscow's offensive on Ukraine.

A Russian court in February sentenced veteran human rights advocate Orlov, 70, to two and a half years in prison for "discrediting" Russia's army after he spoke out against the military campaign and the Kremlin.

"His health is threatened with irreversible consequences," Memorial said in a statement Monday.

It said he was losing his hearing and had become unwell due to "inhumane treatment" by Russian authorities.

"For more than two weeks, his lawyer has not been able to talk with him confidentially," the group added.

This is thwarting Orlov's preparations for an appeal against his sentence, they said.

Orlov is a key figure in the Nobel Prize-winning Memorial group, Russia's leading human rights organization, and a key pillar of the country's independent civil society.

Created in the late 1980s, the Memorial established itself by preserving the memory of victims of communist repression and by campaigning against rights violations.

Russian authorities officially disbanded the organization in late 2021 amid an already tightening repression.

Orlov decided to stay in Russia even as the Kremlin accelerated its crackdown on critical voices after launching its assault on Ukraine.