Some captured members of Ukraine's Azov Regiment will face trial, the speaker of Russia's lower house of parliament said on Thursday, a day after Russia's top court postponed a decision on whether to brand the unit as a terrorist entity.

Vyacheslav Volodin, an ally of President Vladimir Putin, said Russia would carry out a "substantial investigation" into every member of Azov to find out "who was involved in what."

Those who are determined to have "blood on their hands" will face trial, the State Duma speaker said in a post on Telegram.

The Azov Regiment, part of Ukraine's National Guard, was founded in 2014 as one of many volunteer militias to fight pro-Russian separatists in Ukraine's eastern Donbas region. Its fighters were feted as national heroes for defying Russian forces who laid seige on the city of Mariupol for weeks.

The militia originally emerged from a political organisation that championed white nationalist far-right ideas.

Kyiv says it has been reformed away from its origins and that it has nothing to do with politics. Russia says it is still made up of dangerous far-right neo-Nazis and made destroying the unit a war aim.

Some 43 members of the regiment were released on Wednesday in a prisoner exchange between Russia and Ukraine, raising hopes in Kyiv that Russia could be prepared to release more of the fighters.

Volodin said that Wednesday's exchange concerned mainly "the seriously wounded and those unable to participate in hostilities."

Russia's defence ministry said on Thursday that more than 6,000 Ukrainian servicemen had been captured or surrendered since the start of the conflict.

On Wednesday, Russia's Supreme Court postponed for the second time a hearing on whether to designate the Azov Regiment as a terrorist entity - a step that could pave the way for trials.

Relatives of the Ukrainian fighters have appealed for their rights to be protected under the 1949 Geneva Conventions, which state that prisoners of war should be humanely treated and released and repatriated without delay after active hostilities cease.

  - Reuters