Russia's top court declared Ukraine's Azov Regiment a terrorist group, paving the way for captured soldiers to be tried under stringent anti-terror laws and will be jailed for up to 20 years.

The Azov Regiment, which has far-right and ultra-nationalist roots, has been one of the most prominent Ukrainian military formations fighting against Russia in eastern Ukraine. Having begun as a paramilitary unit fighting against pro-Russian rebels in 2014, it was later integrated into Ukraine's national guard.

Russia has regularly cited Azov in support of its assertion that Ukraine is controlled by "fascists." Russian state media has compared Azov fighters to World War Two-era Nazis, whose defeat by the Soviet Union remains a core part of Russian national identity.

Ukraine's Azov regiment issued a response on Tuesday to its designation as a "terrorist" group by Russia's highest court, decrying the move as a justification for Russia to commit war crimes.

In a post on its Telegram page, the Azov regiment also called for the U.S. State Department to declare Russia a "terrorist state" in response to alleged war crimes by Moscow during its invasion of Ukraine.

Previously based in the eastern Ukrainian port city of Mariupol, many of the regiment's personnel were captured by Russian forces when the city fell in May after an almost three-month-long siege.

Officials in the Donetsk People's Republic, the Russian-backed entity which claims Mariupol as part of its territory, said in May that captured Azov Regiment fighters could face the death penalty under the self-proclaimed republic's laws.

Last week, Russian Embassy in London said in a Twitter post that captive Azov personnel should be hanged and that they "deserve a humiliating death."