The expulsion of the Russians was reported earlier by Sky News, which said Moscow's mission to the alliance headquarters in Brussels would be halved "in response to suspected malign Russian activities, including killings and espionage".
Reuters was not immediately able to confirm the reasons cited by Sky News for the reduction of the Russian delegation.
There was no immediate comment from the Kremlin or the Russian foreign ministry.
The West's ties with Russia remain strained over everything from Ukraine to alleged Russian election meddling to the 2018 poisoning with a nerve agent in England of Russian double agent Sergei Skripal and his daughter.
The Interfax news agency cited Leonid Slutsky, head of the Russian lower house of parliament's international affairs committee, as saying that Moscow would retaliate, but not necessarily in kind.