WASHINGTON — A group of elite Russian military intelligence officers, including some of those who planned the poisoning of a defector in Britain, have been operating out of picturesque villages in the French Alps, Western intelligence officials tell NBC News.

Confirming a report in France's Le Monde daily that could have been ripped from the pages of a John le Carré spy novel, the officials said European and American intelligence agencies had been tracking up to 15 members of the Russian military intelligence agency known as the GRU who had lived for a time in France. Le Monde published what it said were the names of the officers, some of which had already been published by Bellingcat, an open source investigative group.

Le Monde reported that among the Russians who stayed in France's Haute-Savoie department in the Alps were Alexander Petrov and Ruslan Boshirov — the alleged cover names of the two GRU agents accused of carrying out the attack on Skripal. French officials told the newspaper they considered the area to have been the unit's rear base for covert operations in Europe.

"It's good that our European partners are taking the Russian threat so seriously," said Marc Polymeropoulos, a former senior CIA officer with expertise in Europe and Russia. "Russia behaves at times like an outlaw regime, attempting to kill dissidents abroad and fomenting unrest in European democracies. It takes the sustained efforts of European security services working together to counter this threat."

This week, Germany expelled two Russian diplomats after authorities concluded Russian agents orchestrated an execution-style killing in Berlin of a 40-year-old Russian-Georgian citizen who has been designated as a terrorist by Moscow.

The victim had fought with a Georgian unit defending South Ossetia during the 2008 Georgia-Russia war, German prosecutors said in a statement.

British authorities say Russian agents used a nerve agent in March 2018 to poison a former Russian spy, Sergei Skripal. His daughter was also stricken. Both have since recovered.

In September, the Crown Prosecution Service charged the Russian GRU officers who used the aliases Petrov and Boshirov with attempted murder, conspiracy and possession of a nerve agent for the Skripal attack.

In addition to the Skripals, the nerve agent harmed three others who came in contact with it. One of them, Dawn Sturgess, died.

Russia has denied involvement in the poisoning, and Russian officials have called the allegations propaganda.

Emergency workers at the scene where a man and a woman were found in 2018 in critical condition at The Maltings shopping center in Salisbury, England. Ben Stanstall / AFP - Getty Images file

Given Europe's open borders, Russian operatives don't have much problem getting in and out to undertake spying operations, intelligence officials say. What's notable about the revelations about the Alps base is that French authorities decided to make the information public.

The FBI is tracking as many as 200 Russian operatives in the United States, current and former officials say. But some slip through unnoticed, as was demonstrated by special counsel Robert Mueller's indictments accusing Russians of using social media propaganda to influence the 2016 election. The indictment said some of the defendants traveled to the U.S. under false pretenses "for the purposes of collecting intelligence." But their scheme was not discovered until much later.

Michele Neubert reported from London.