The TASS and Interfax news agencies as well as the Baza and Nezygar online news resources, quoting unnamed sources, said on August 29 that Sentsov had been transferred to Moscow's Detention Facility No. 2, also known as the Butyrskaya prison, from a penitentiary in the town of Labytnagi in the Yamalo-Nenets Autonomous District.

Sentsov has been imprisoned in Russia since opposing Moscow's takeover of his native Crimea in March 2014.

He was sentenced to 20 years in prison after being convicted of terrorism in a trial criticized by human rights groups and Western governments as politically motivated.

Reports about Sentsov's transfer to Moscow come a day after a court in Ukraine ordered jailed Russian journalist Kirill Vyshinsky to be freed on his own recognizance and released from custody before his trial on treason charges.

Unconfirmed reports said in recent days that Russia and Ukraine were going to exchange Sentsov for Vyshinsky.

Sentsov's mother, Lyudmyla Sentsova, told RFE/RL on August 29 that her son skipped their usual end-of-month telephone conversation, which had never happened before.

"Yesterday, August 28, was exactly one month since he called me. He usually calls for 20 minutes exactly once a month. I was waiting for his call today as well, but he did not call, " Sentsova said, adding that for the first time in five years she had received a postcard from her son on August 12, in which he congratulated her on her birthday and wrote "trust me, we will hug each other soon."

Sentsova also said that she has no information about her son's current location.

Last year, Sentsov went on a 145-day hunger strike protesting his imprisonment and demanding the immediate release of all Ukrainian citizens held in Russia on politically motivated charges.

In December, the European Parliament awarded Sentsov its 2018 Sakharov Prize for Freedom of Thought.

The Moscow-based Memorial Human Rights Center has recognized Sentsov as a political prisoner.

Kyiv and Moscow have held talks in recent weeks on a prisoner swap that could include dozens of prisoners.

Ukraine's Opposition Platform-For Life party leader Viktor Medvedchuk, a close Kremlin ally, said on August 29 that the talks are ongoing.

"I know that such negotiations are ongoing, but there is no final solution. Also, the question of the person you named, Sentsov, has not been resolved," Medvedchuk said during the inaugural session of parliament.

Ukrainian President Volodymyr Zelenskiy said last week that he hopes to see the "first results of the prisoner swap" soon.

With reporting by TASS, Interfax, Baza, and Nezygar