French prosecutors say three people have been placed in custody on suspicion of spying for Russia and promoting its wartime propaganda, while a fourth suspect has been placed under strict police supervision.

Prosecutors said on Wednesday that the suspects held in detention include a 40-year-old Russian man filmed putting up pro-Moscow posters on the iconic Arc de Triomphe in Paris; a 40-year-old Russian-born woman known as the head of the French-Russian SOS Donbass group, and a 63-year-old man from a northern suburb of Paris.

A fourth suspect, 58, was spared pre-trial detention but placed under strict supervision and ordered to report to the police once a week.

The woman leading SOS Donbass, which presents itself as a humanitarian group helping civilians in Ukraine’s eastern Donbas region, had been monitored by France’s domestic intelligence service since early this year, the General Directorate for Internal Security, which is known by its French-language acronym DGSI, said.

After detecting “actions likely to harm the fundamental interests of the nation”, the DGSI opened a judicial inquiry in March into suspected offences by the woman, including “collusion with a foreign power”, a charge that carries up to 10 years in prison.

French counterespionage officials believe the woman was trying to collect economic information from French company executives.

The detentions come as fears of Russian espionage grow across Europe, with countries pointing the finger at Moscow’s spy services amid sabotage attacks as Russia’s war in Ukraine grinds on.

Last month, police in the United Kingdom arrested three men in their 40s under the 2023 National Security Act, accusing them of assisting Russia’s foreign intelligence service.

In a separate case, two young ringleaders of a group were given lengthy sentences on charges of carrying out arson attacks in London on behalf of the Wagner Group – the Russian state-funded mercenary force.

In March, three Bulgarian nationals were convicted in London of belonging to a Russian spy unit that conducted surveillance for the Kremlin on a US military base and on individuals targeted by Moscow.

Tensions escalated further last week when UK Defence Secretary John Healey said the crew on board the Russian spy ship Yantar had aimed lasers at the pilots of British surveillance aircraft sent to observe the vessel in waters off Scotland.

Healey condemned the move as “reckless and dangerous” and said the UK would respond to any territorial incursions.

“We have military options ready should the Yantar change course”, he said.

The Kremlin has rejected all allegations, accusing London of blaming Russia whenever something “bad” happens in the UK.

Switzerland’s Federal Intelligence Service (FIS) also reports that the country has become an espionage hotspot since Moscow’s invasion of Ukraine. The FIS estimated in 2023 that at least a third of Russia’s 220 accredited officials in the country were likely spies.

The UK Ministry of Defence said on Sunday that a British patrol vessel intercepted a Russian corvette and tanker in the English Channel, noting Russian naval movements near UK waters had risen by 30 percent in two years.

NATO has also stepped up air patrols along Ukraine’s border following suspected Russian airspace incursions and drone sightings in several member states, including Poland, where authorities last month arrested eight people suspected of espionage and sabotage on behalf of Russia.

In December 2024, Finland seized a Russia-linked ship suspected of deliberately damaging an undersea cable between Finland and Estonia.