The trial of a Russian citizen charged with "participation in an aggressive war in the Donbass" began in Ukraine on Friday, the Mediazona news website reported Monday.
Andrei Langer, 34, is also charged with illegal weapon possession and joining a terrorist organization.
He faces up to 15 years in jail if convicted, according to Ukrainian prosecutor Ruslan Kravchenko.
Langer joined the army of the so-called Donetsk Peoples' Republic as a volunteer in July 2015 after he was released from jail, where he had served two years for desertion from the Russian army.