"We call on the leaders of the European Union to step up pressure on Russian authorities and President Putin, demanding immediate release of Oleg Sentsov and other Ukrainian nationals held under politically motivated charges in Russia's prisons," reads their letter addressed to European Council President Donald Tusk, European Parliament President Antonio Tajani, European Commission President Jean-Claude Juncker and Vice President of the European Commission Federica Mogherini.

The letter also calls on the European Union to identify persons who contributed to the detention and prosecution of Sentsov and subject them to sanctions for gross violations of human rights.

Sentsov has been on a hunger strike since May 14. He was convicted on terrorism charges and sentenced to 20 years in prison, serving in Russia's north.

Baltic politicians demand that Moscow releases dozens of Ukrainian political prisoners held in Russian prisons.