Oleh Sentsov / Photo from UNIAN

He also addressed overall conditions in Russian penitentiary facilities and spoke about the confessions of the Russian military involved in seizing from Ukraine the Crimea peninsula, the Voice of America reports.

According to the Ukrainian film director, the Russian president grabbed Crimea when Ukrainian citizens toppled the regime of "Putin's direct subordinate" Viktor Yanukovych.

Speaking about grave conditions in Russian prisons, Sentsov said: "One of the kinds of torture in Russian prisons is strangulation with a plastic bag. I had seen this in American movies, and I couldn't understand why people break down. But your primal instinct kicks in when you're deprived of the ability to breathe. And then you are overwhelmed with this extraordinary animal fear, which is hard to fight," Sentsov said.

Read alsoEx-political prisoner Sentsov says not interested in politics

The Ukrainian film maker added that it was in prison where he met with a Russian military intelligence operative who had been involved in the annexation of Crimea: "In prison I met a convicted GRU officer who told in detail how the Crimea seizure unfolded. He took part in this. Then this officer fought in Donbas – he said that it was the Russians who were most brutal towards the Ukrainians."

Earlier, Sentsov called on the United Nations to fight for the release of the rest of Ukrainian captives held in Russia.

Sentsov, who was freed by Russia in a prisoner swap with Ukraine in September, is on a trip to the U.S. tp meet with State Department officials, members of Congress, and attend a meeting of the U.S. Helsinki Commission, RFE/RL reported earlier.

Russia still holds 96 political prisoners, most of whom were detained in Crimea, including 69 Crimean Tatars, according to Ukrainian NGOs. Anywhere from 101 to 184 are still held in the occupied Donbas, according to NGOs and the Ukrainian government.