A senior French army officer is under investigation for a suspected “breach of security”, the armed forces minister has confirmed.
Florence Parly gave no further details but sources told French media the officer was stationed with Nato and based in Italy.
The lieutenant-colonel was suspected of passing sensitive information to Russian intelligence, Europe 1 reports.
He has been remanded in custody in Paris, the radio station said.
Ms Parly told Europe 1 it was France that had instigated the legal proceedings in the case and it was now in the hands of prosecutors.
“We have taken all the protective measures that were necessary,” she said.
A judicial source told Agence France-Presse the officer had been remanded and was being investigated over “intelligence with a foreign power that undermines the fundamental interests of the nation”.
Europe 1 said the officer involved had distant family roots in Russia and spoke the language fluently.
It said he worked at a command centre near Naples.
The officer was arrested 10 days ago as he prepared to return to Italy from holiday in France, it said.
The latest arrest comes at a time of tension among Nato partners, stemming mainly from questions about the continued commitment to the alliance of the US under President Donald Trump.
Last November, French President Emmanuel Macron described Nato as “brain dead” because of the waning US commitment.
The latest tensions have arisen over gas and oil in the eastern Mediterranean. Nato members Greece and Turkey are at loggerheads, with France appearing to side with the Greeks.