MOSCOW - Ukrainian filmmaker Oleg Sentsov, who refused food for more than 140 days in a Russian jail, halted his high-profile protest on Friday after President Vladimir Putin rejected his demand to free Ukrainian prisoners.

Russia's most famous prisoner had spent 144 days without solid food but called off his hunger strike after a months-long, star-studded campaign to secure his release failed to press Moscow into action.

"Convict Oleg Sentsov has agreed in writing to consume food," the Russian prison service said in a statement.

Moscow's best nutritionists have developed a special diet to help the 42-year-old resume eating solids and ease out of his fast "without complications", it said.

There was no immediate comment from Sentsov's lawyer Dmitry Dinze.

Deputy head of the prison service, Valery Maksimenko, said Sentsov was currently in a prison hospital but would be transferred back to his barracks once he gets better.

"He is receiving normal, diet food," he told Russian independent TV channel Dozhd, adding the prisoner had until now eaten "pastes from tubes - like a cosmonaut".

Maksimenko told Russian journalists he was grateful to local doctors and lawyers who persuaded Sentsov that "he needs to live, that life goes on".

"He is young, maybe he will become a famous director," Maksimenko told Dozhd. "Let him live," he said, adding that the activist had called off his hunger strike on Friday.

The director is best known for his film "Gamer", which screened to critical acclaim at the Rotterdam Film Festival in 2012.

Sentsov is serving a 20-year sentence in a prison in the Russian Arctic after being convicted on terrorism charges over an alleged arson plot in Crimea.

A vocal Kremlin critic, Sentsov was detained in 2014 after Russia annexed Crimea from Ukraine.

Supporters say Russia wanted to make an example of him with the stiff sentence. The filmmaker has denied the charges. He has two children, one of whom is autistic.

Kiev said it needed to hear from Moscow before it could comment. "We need official information from Russia which tortures and keeps Oleg prisoner, and not its official spin doctors," deputy parliament speaker Iryna Gerashchenko wrote on Facebook.

Sentsov began his hunger strike on May 14. He had been sustained with a glucose drip, some nutritional supplements and water.

Late last month the prison service released a rare photo of a tired-looking Sentsov undergoing a medical checkup, his ribs clearly visible through his skin.

The film director had timed his protest to coincide with the World Cup that Russia hosted in June and July in an effort to exert maximum pressure during Russia's chance to shine on the world stage.

The Kremlin has not budged however.

Moscow has refused to free Ukrainian prisoners and refused to free Sentsov, saying he must ask Putin for a pardon for his release to be considered.

The Ukrainian director has refused to do so.

Supporters have waged a global campaign, urging the Kremlin to release Sentsov and staging protests in dozens of cities in Europe, the United States and the Middle East.

Scores of celebrities have taken part, including bestselling US author Stephen King, Hollywood actor Johnny Depp and French actress Isabelle Huppert.

In a letter to Putin ahead of the World Cup, dozens of prominent authors including Margaret Atwood and Ian McEwan had called for Sentsov's release.

Supporters in Ukraine and Russia have long called for Sentsov to start eating again, saying they needed him alive. Several activists fasted in solidarity with him but quickly stopped, admitting they were not strong enough.