The state Investigative Committee said the man was suspected, over a period of several years, to have "purposefully collected information in the field of military and military-technical activities of the Russian Federation".

It added: "This information, when obtained by foreign sources, can be used against the security of the state. For these purposes, he repeatedly visited the territory of Russia, including the city of Moscow, where he held meetings with citizens of the Russian Federation."

Investigators said the man, whom they did not identify, would be charged shortly under a Russian law on failing to register as a "foreign agent", which carries a punishment of up to five years in prison.

No comment was immediately available from the French Foreign Ministry in Paris.

The TASS state news agency identified the man as an employee of the Centre for Humanitarian Dialogue (HD), a Swiss-based nonprofit, whose website says it has operated since 1999 to "help prevent, mitigate and resolve armed conflict through dialogue and mediation" in countries such as Spain, the Philippines and Libya.

HD did not immediately reply to a request to confirm the arrested man was its employee. It was not immediately clear if the organisation has operations or staff in Russia.

MASKED MEN IN RESTAURANT

A short video clip released by the Investigative Committee showed a man in jeans and a black shirt being detained in a restaurant by security officials with masks over their faces, escorted into a van and then being walked into a building. The suspect's face was blurred out.

Reuters was able to identify the restaurant shown in the clip as a cafe called Akademia in central Moscow.

Westerners arrested under Russian security laws since the start of the Ukraine war have found themselves caught up in the deepest crisis in relations between Moscow and the West for more than 60 years.

Last year, U.S. reporter Evan Gershkovich was arrested on suspicion of trying to obtain military secrets and charged with espionage, which carries a sentence of up to 20 years. Another American, Paul Whelan, was convicted in 2020 and is serving a 16-year sentence for spying. The United States has designated both as "wrongfully detained" and is seeking their release.

Gordon Black, a U.S. soldier, was detained on May 2 by police in the far eastern city of Vladivostok and went on trial on Thursday on suspicion of stealing from his Russian girlfriend and threatening to kill her.

Russian-American journalist Alsu Kurmasheva was arrested last October and is awaiting trial on charges including failing to register as a "foreign agent" - a designation that Russia applies to people and organisations it deems to be using foreign funding to conduct political activity, and which it has used extensively to clamp down on dissent.