A source said the man was from the Donbas region of eastern Ukraine, part of which has been occupied by pro-Russian and Russian forces since 2014.
He appeared to have a “pro-Russian attitude” after fighting “for two years for the Russian army”, the source added.
He was charged as France was hosting Joe Biden, the US president, and Volodymyr Zelensky, the Ukrainian president, following commemorations on Thursday of the 80th anniversary of D-Day landings.
French prosecutors said the suspect had been remanded in custody pending the investigation.
French man held in Moscow
Meanwhile, Laurent Vinatier, a 47-year-old French national who works for a Swiss conflict mediation NGO, was arrested in Moscow and charged with breaching Russia’s “foreign agents” law. The charges carry up to five years in jail.
Mr Vinatier, who was held in a metal cage at the back of the courtroom, accepted the accusations against him.
“I have accepted that I am guilty,” he told the court. “I present my apologies.”
The judge ordered Mr Vinatier to be sent to jail until at least Aug 5, pending trial.
Investigators accuse the Frenchman of gathering military information that could be used against Russia by foreign states.
Under Russian law, people who collect, report or share information related to Russia’s military or security services are required to register as “foreign agents”.
The law has typically been used to target Russian citizens and domestic critics of the Kremlin, rather than foreign citizens.
On Thursday, Emmanuel Macron said he was aware of Mr Vinatier’s arrest and added that he had no connections with the French government.
“It is one of our citizens working for a Swiss NGO … it was in no way someone who was working for France,” said the French president.
Mr Macron called on Russia to “clarify all the facts and to release him as soon as possible”.