The secretary of the Council of Europe rights group on Wednesday called for Russia to release the Ukrainian film director Oleg Sentsov, who is currently on hunger strike as he serves a 20-year sentence.
A number of influential Russian and foreign figures, including the horror writer Stephen King and Oscar-nominated director Andrei Zvyagintsev, have in recent days also demanded the film-maker's release.
A vocal Kremlin critic, Sentsov was detained in Crimea in 2014 after Russia annexed the peninsula from Ukraine.
"I think Sentsov must be liberated for humanitarian reasons. He should be pardoned for humanitarian reasons," Thorbjorn Jagland said in comments reported by the Interfax news agency.
"If it is necessary to file a request to pardon him, I will do it with pleasure in accordance with the European Convention on Human Rights," he said, in remarks translated into Russian.
Supporters of the film-maker say Moscow set out to make an example of him with a harsh sentence on charges of masterminding arson attacks in Crimea, which the 41-year-old denies.
On Tuesday 12 big names in the Russian arts, including Zvyagintsev and fellow filmmaker Alexander Sokurov, director of "Russian Ark", called for Putin to pardon Sentsov.
Kremlin spokesman Dmitry Peskov told journalists the director would have to ask for the pardon himself before it could be considered.
Moscow's top rights official Tatyana Moskalkova said Wednesday she was receiving daily reports from Sentov's prison camp in the far north after he went on hunger strike on May 14, announcing he was prepared to die in jail.
"He is receiving nutrition from a drip filled with all the vitamins twice a day," she said in comments reported by the RIA Novosti news agency, adding that he had not lost weight.