"Based on evidence collected by the Federal Security Service of Russia, the Southern District Military Court recognized the Georgian National Legion paramilitary formation ... as a terrorist organization," the FSB said in a statement.
Due to the court decision and federal law, its activities are prohibited on the territory of Russia, the statement added.
"Members of the legion participate in hostilities on the side of the Ukrainian armed forces, and are also involved in organizing sabotage and terrorist attacks on Russian territory, torturing military personnel of the Russian armed forces and killing civilians," the FSB said.
The militia were trained by instructors from New Zealand, Sweden and other NATO countries, a captured member of the organization said in footage distributed by the security services. The training included tactics, drone operations and tactical maneuvers, the member added.
"They [the Legion's commanders] mostly marauded instead of capturing positions. They also gave obscure orders like shooting civilians who were, allegedly, saboteurs," the member said, adding that the militia were persuaded that some of the civilians had been given grenades to use against the Ukrainian military.
The Georgian National Legion is reportedly the largest military formation assembled on the basis of nationality operating in Ukraine, the FSB said, adding that the Legion was established in 2014 under the direction of the Ukrainian Defense Ministry's Main Directorate of Intelligence.