“The criminal case was opened against Roman Zheleznov previously convicted for theft and hooliganism,” spokesman of Russian Investigative Committee Vladimir Markin told TASS.
Zheleznov has been identified as an accomplice of Maxim "Tesak" Martsinkevich, sentenced to five years in jail in August for extremism and inciting inter-ethnic strife.
A source close to the case told TASS that Zheleznov was in Kiev and that searches had been made of his living quarters in Moscow.
Investigators say the fugitive arrived in Ukraine in July, meeting battalion commanders, joining up and agreeing to fight in the troubled Donbass region for a monthly pay of 4,000 Ukrainian hryvnias ($308).
From July 9 to July 26, he underwent training and after receiving personal weapons and a uniform, is said to have arrived for military duty on the territory of the self-proclaimed Donetsk People’s Republic.
It's planned to put Zheleznov on the international wanted list.