"Fifty-six people were released, 55 of them servicemen... Viktor Medvedchuk was also released from captivity," Denis Pushilin told the state-run RIA Novosti news agency, referring to the former Ukrainian lawmaker, who was accused of high treason.

Medvedchuk, 68, is one of Ukraine's richest people and says Putin is the godfather to his youngest daughter.

He was captured by Ukraine's special forces in mid-April after escaping from house arrest.

Russia's defence ministry said earlier that the released servicemen have been transported to Russia and "are in medical institutions of Russia's defence ministry".

It claimed in a statement that the released prisoners were "in mortal danger" while in captivity.

It did not mention the whereabouts of Medvedchuk.

The defence ministry released a video of what it said were the freed servicemen, showing men in military uniform disembarking a plane during the night.

Following the exchange announced on Wednesday, Ukraine received 215 people, including fighters who led the defence of Mariupol's Azovstal steelworks that became an icon of Ukrainian resistance.

"We were ready to accept such a price, we gave away 215 people, including some of them... war criminals," Pushilin said, likely referring to fighters of the Azov regiment, a former volunteer battalion that was incorporated into Ukraine's army.

Azov fighters are hate figures for Russia's state media and officials, who demonise them as "neo-Nazis" and have called to put them on trial.

"But the task was to return our guys as soon as possible," Pushilin added.

Also in the statement, the defence ministry's daily briefing, Moscow accused Kyiv of "provocations" at the Russian-held Zaporizhzhia nuclear power plant in southern Ukraine "aimed at creating a threat of a man-made disaster".

It said that over the past 24 hours, Ukraine repeatedly shelled the nearby city of Energodar and the territory near the plant, Europe's largest nuclear facility, adding that radiation levels were "normal".

The plant has been a hot spot for concerns of a nuclear incident after tit-for-tat claims of attacks there.