France -- French authorities have arrested three people on suspicion of spying for Russia and acting to promote its war propaganda, prosecutors said Tuesday.

The arrests, which are part of a probe into the French-Russian association SOS Donbass, come as fears of Russian interference across Europe run high due to tensions over Moscow's war on Ukraine.

One of the detainees, a 40-year-old Russian man, was seen hanging up pro-Russian posters on the Arc de Triomphe in Paris on video surveillance footage in September, the city prosecutors' office said.

The man then reported by telephone to the head of SOS Donbass, a 40-year-old woman who was suspected by French counter-espionage officials of trying to obtain financial information from French business executives.

The Russia-born woman had been on the radar of France's domestic intelligence service, the General Directorate for Internal Security (DGSI), since at least the beginning of the year.

After the DGSI detected "actions likely to harm the fundamental interests of the nation" relating to the woman, an investigating magistrate was appointed in March to probe various suspected offenses including "collusion with a foreign power," which is punishable by 10 years in jail.

SOS Donbass presents itself as a humanitarian organization offering aid to civilians in the eastern Donbas region of Ukraine, which is partially under Moscow's control.

The third person in custody is a 63-year-old man born in the northern Paris suburb of Seine-Saint-Denis.

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