MOSCOW - Russia's prison service on Saturday claimed that Ukrainian hunger-striker Oleg Sentsov is not underweight, denying a statement by his lawyer that he has lost 30 kilograms in weight.

Sentsov, a 42-year-old filmmaker and Kremlin opponent convicted on terrorism charges, has been on hunger strike in a far northern Russian prison for almost three months, demanding Moscow release Ukrainian political prisoners.

The prison service stated in a bulletin on Sentsov's health on its website that: "As of today he is not seen to be underweight and a worsening in his state of health is not observed."

The statement came after French President Emmanuel Macron raised Sentsov's situation with Russian counterpart Vladimir Putin in a phone call on Friday.

Putin promised "to respond and quickly release details on Sentsov's health," the French presidency said.

Sentsov's legal representative, respected rights lawyer Dmitry Dinze, said in a Friday interview that Sentsov is "ready to die" and after visiting him on Tuesday told AFP he had lost 30 kilograms.

The prison service released a statement on Saturday on its website specifically referring to the 30-kilogram weight loss figure, saying "this information does not correspond to reality." The service said it "is taking all the necessary measures" to maintain Sentsov's state of health.

The prisoner takes nutritional supplements to protect his heart and nervous system, it said.

He does not require emergency hospitalisation, it said, calling his state of health "satisfactory."

Sentsov is serving a 20-year sentence after he was convicted of arson attacks on pro-Moscow party offices in Crimea after its 2014 annexation from Ukraine.

He declared a hunger strike on May 14. He has refused to be transferred to a civilian hospital or to cease the hunger strike.

The Russian prison authorities and the country's rights ombudswoman have consistently played down fears over his state of health.

Western governments and celebrities including Pedro Almodovar, Johnny Depp and Stephen King have repeatedly urged the Kremlin to release Sentsov.

Sentsov's mother has urged Putin to issue him with a pardon and the Ukrainian President Petro Poroshenko has discussed the possibility of a prisoner swap with the Russian leader, but supporters are losing hope that he will be released.

The US Embassy said Friday that it is "seriously concerned" about Sentsov's health and called for Russia to release him and "his fellow Ukrainian prisoners" on humanitarian grounds.