Ukrainian filmmaker Oleh Sentsov, who opposed Moscow's 2014 annexation of Crimea and is now on hunger strike in a Russian prison, has agreed to receive medical treatment, Russia's prison authorities say.

The Federal Penitentiary Service's branch for in the far-northern Yamalo-Nenets Autonomous Region said in a statement on May 28 that Sentsov was under the supervision of medical workers in the correctional facility where he is serving his sentence.

"At the moment, his state of health is satisfactory," it also said.

Sentsov, who is a native of the Crimean Peninsula, is currently serving a 20-year prison term after being convicted on terrorism charges that he and human rights groups say were politically motivated.

Sentsov said he began a hunger strike on May 14, demanding the release of 64 Ukrainian citizens that he considers to be political prisoners in Russia.

Lawyer Dmitry Dinze said Sentsov had told him he had timed his hunger strike to correspond with Russia's hosting of the 2018 soccer World Cup championship from June 14 to July 15.

The Save Oleg Sentsov activist group last week announced a global campaign on June 1-2 -- ahead of the World Cup -- to demand the release of the Ukrainian filmmaker.

Sentsov, 41, was arrested in May 2014 on suspicion of planning fire bombings of pro-Russian organizations in Crimea. A Russian court convicted him on multiple terrorism charges in August 2014.

Sentsov has denied all charges against him, saying that a "trial by occupiers cannot be fair by definition."

The prominent Russian human rights group Memorial has recognized Sentsov as a political prisoner, and international rights organizations have called for his release.