The Haute Savoie region in France has been named as a base camp for Russian spies specialising in targeted killings around Europe.

According to French newspaper Le Monde, British, Swiss, French, and American intelligence services had been tracking up to fifteen suspected agents, who all passed through mountains near the Swiss and Italian borders between 2014 and 2018.

The list includes some of those who planned the poisoning of ex Russian intelligence agent Sergei Skripal and his daughter in March 2018 in Salisbury, in the south of England.

"It's the first time since the second world war that a chemical attack was ordered by one state to another in Europe," journalist for Le Monde Jacques Follorou told Euronews. "In this case from Russia to England."

"There was a reaction of solidarity between France, Great Britain and Switzerland and their allies like the USA. For them, the red line had been crossed and it was not acceptable."

A list of 15 members of the Russian military intelligence agency known as the GRU emerged from this hunt, some of which had already been published by Bellingcat, an open-source investigative group.

Among the Russians who stayed in the region were Alexander Petrov and Ruslan Boshirov, the alleged cover names of the two GRU agents accused of carrying out the attack on Skripal, Le Monde reported.

The Haute-Savoie department in the Alps was used as a logistics base for its covert operations in Europe.

"The most logic hypothesis is that Haute-Savoie was considered an easy place to access; it's near Geneva, allowing them to easily and discreetly move across western Europe," Follorou said.

"The cities of Megeve, Chamonix, Annemasse and Geneva are also very popular places for Russian tourists."

And there is another reason that French intelligence services did not detect any illegal operation between that period; the agents were discreet and didn't operate in France to avoid suspicion.

Instead, they carried out undercover operations in the rest of Europe; in the UK, Bulgaria, Moldova, and Montenegro.

But now, they appear to have gone quiet.

"Many of them are the subject of European arrest warrants," Follorou said. "You now have a number of people whose names have been made public. It is now extremely complicated for them to move and act as clandestine agents. They are not clandestine any more."

"This service obviously had to be stopped, and will probably be reorganised by Russia."

WATCH: Katy Dartford reports on the purported Haute-Savoie 'spies':