The four intelligence officers detained by Dutch security services are part of an elite military unit that runs high-risk cyber espionage operations out of a 19th-century barracks in central Moscow.

Unit 26165 - named by US prosecutors as “GRU 85 Main Special Service Center” - is home to the Russian military's best mathematical minds and is believed to have run the hacking campaign to influence the 2016 US elections.

Dutch and British authorities believe the unit was also involved in attempts to hack the investigations into doping of Russian athletes and the shooting down of Malaysian airlines flight MH17.

"Unit 26165 seems to be quite old, as far as we can tell from the open sources available," said Andrei Soldatov, the author of The Red Web and an expert on Russian cyber operations. "It has probably existed since the '70s, and was known as a unit dealing with cryptography."

The unit is believed to come under the command of the GRU's sixth directorate - the part of the Russian military intelligence agency that deals with signals intelligence.