First came a destabilization campaign in Moldova, followed by the poisoning of an arms dealer in Bulgaria and then a thwarted coup in Montenegro. Last year, there was an attempt to assassinate a former Russian spy in Britain using a nerve agent. Though the operations bore the fingerprints of Russia’s intelligence services, the authorities initially saw them as isolated, unconnected attacks.

Western security officials have now concluded that these operations, and potentially many others, are part of a coordinated and ongoing campaign to destabilize Europe, executed by an elite unit inside the Russian intelligence system skilled in subversion, sabotage and assassination.

The group, known as Unit 29155, has operated for at least a decade, yet Western officials only recently discovered it. Intelligence officials in four Western countries say it is unclear how often the unit is mobilized and warn that it is impossible to know when and where its operatives will strike.

The purpose of Unit 29155, which has not been previously reported, underscores the degree to which the Russian president, Vladimir V. Putin, is actively fighting the West with his brand of so-called hybrid warfare — a blend of propaganda, hacking attacks and disinformation — as well as open military confrontation.