The Russian Supreme Court on Tuesday declared Ukraine's Azov Regiment a terrorist organization banned in Russia, a designation that may expose Ukrainian prisoners of war held by Moscow to terrorism charges.

Azov, which played a key part in the defense of the southeastern Ukrainian city of Mariupol, has been repeatedly portrayed by Russian officials and state media as a Nazi formation allegedly committing atrocities against Ukraine's civilians. No evidence has so far surfaced to back up these assertions.

The Azov Regiment is a unit within Ukraine's National Guard. It grew out of a group called the Azov Battalion, formed in 2014 as one of many volunteer brigades that rose to bolster Ukraine's underfunded and questionably led military in the fight against Russia-backed separatists in eastern Ukraine.

The Azov Battalion drew its initial fighters from far-right circles and elicited criticism for some of its tactics. Its current members rejected accusations of nationalism and radicalism.

Russia's Prosecutor General's office filed a motion to designate the regiment as a terrorist organization in May.

Scores of Azov fighters are being held captive by Moscow. The Russian authorities have opened multiple criminal cases against them, accusing them of killing civilians.

Last week, dozens of Ukrainian POWs were killed in a strike on a barracks at a penal colony in Olenivka, an eastern town controlled by pro-Russian separatists. Moscow and Kyiv have have traded blame for the strike, with Kyiv saying Russia blew up the barracks to cover up torture against the POWs.