Russia has released 215 Ukrainians it took prisoner after a protracted battle for the port city of Mariupol earlier this year, including top military leaders, a senior official in Kyiv said on Wednesday, Reuters reported.

The freed prisoners include the commander and deputy commander of the Azov battalion that did much of the fighting, said Andriy Yermak, the head of Ukrainian President Volodymyr Zelenskyy's office.

The move is unexpected, since Russian-backed separatists last month said there would be a trial of Azov personnel, who Moscow describes as Nazis. Ukraine denies the charge.

In a statement, Yermak said the freed prisoners included Azov commander Lieutenant Colonel Denys Prokopenko and his deputy, Svyatoslav Palamar.

Also at liberty is Serhiy Volynsky, the commander of the 36th Marine Brigade of the Ukrainian Armed Forces.

The three men had helped lead a dogged weeks-long resistance from the bunkers and tunnels below Mariupol's giant steel works before they and hundreds of Azov fighters surrendered in May to Russian-backed forces.

Yermak said that in return, Kyiv had freed 55 Russian prisoners as well as Viktor Medvedchuk, the leader of a banned pro-Russian party who was facing treason charges.

Public broadcaster Suspline said the exchange had happened near the northern Ukrainian city of Chernihiv.

Earlier in the day, Saudi Arabia said Russia had released 10 foreign prisoners of war captured in Ukraine following mediation by Saudi Crown Prince Mohammed bin Salman.