"Today we are returning 107 of our fighters from Ukrainian dungeons," pro-Moscow leader Denis Pushilin said on Telegram, adding that 65 of those were from eastern Ukraine's separatist territories.

"We will give back the same amount of prisoners to Ukraine," Pushilin said.

The Russian defence ministry said "the released will be sent to Moscow for treatment and rehabilitation."

Ukraine's presidential chief of staff Andriy Yermak confirmed Kyiv received 107 servicemen.

Among them, "74 soldiers defended Azovstal", the steelwork in the port of Mariupol that became the symbol of Ukrainian resistance at the start of Moscow's offensive.

Russia seized most of Mariupol in April after a weeks-long siege but hundreds of Ukrainian soldiers remained holed up in the underground tunnels of the Azovstal steelwork.

The Azovstal soldiers, whose long resistance despite ammunition and supplies shortage earned them praise across Ukraine, surrendered in May.

"Some volunteers (who) flew by helicopter to Mariupol to save lives and fight for the city" were also released, Yermak said.

He posted photos and videos of a dozen released prisoners walking on a bridge, smiling and waving.