NATO has expelled eight members of Russia's mission to the alliance after they were revealed to belong to Moscow's intelligence services and cut the mission's size in half, a NATO official told RFE/RL.
The official said that the alliance had "strengthened our deterrence and defense in response to Russia's aggressive actions," but was still willing to engage in dialogue.
"We can confirm that we have withdrawn the accreditation of eight members of the Russian Mission to NATO, who were undeclared Russian intelligence officers," the official, who spoke under the condition of anonymity, told RFE/RL.
"We can also confirm that we have reduced the number of positions which the Russian Federation can accredit to NATO [from 20] to 10.
"NATO's policy towards Russia remains consistent. We have strengthened our deterrence and defense in response to Russia's aggressive actions, while at the same time we remain open for a meaningful dialogue," the official said.
Relations between NATO and Russia have been tense recently and official contacts have been limited since Moscow annexed Ukraine's Crimean Peninsula in 2014.
The 30-member Western alliance is also concerned over Russia's nuclear missile development, aerial intrusions into NATO airspace, and the buzzing of allied ships by Russian fighter planes.
The main forum for dialogue between the two sides, the NATO-Russia Council, is stalled.
"NATO proposed to hold another meeting of the NATO-Russia Council over 18 months ago, and that proposal stands. The ball is in Russia's court," the official said.
There was no immediate official reaction from Moscow, but a senior Russian lawmaker, Leon Slutsky, told Interfax that Moscow will retaliate against NATO's decision.
Slutsky, head of the lower house of parliament's Foreign Affairs Committee, was quoted as saying that Russia would retaliate, "and not necessarily in a symmetrical way."
NATO's move was the second time the alliance has taken action against Russia on such a scale, after it expelled seven Russian diplomats from the mission following the 2018 poisoning of former Russian military intelligence officer Sergei Skripal in the English city of Salisbury.
Those expulsions were part of mass ejections of Russian intelligence officers across allied countries.
At the time, NATO also reduced the size of the Russian mission to 20 from 30 people.
Skripal and his daughter Yulia nearly died after being exposed to what British authorities later concluded was Novichok, a military-grade nerve agent. A British woman who accidentally came into contact with the substance died.
The latest move, which was apparently agreed on October 5 by all 30 alliance members, is reported to have come after revelations emerged in April about suspected Russian involvement in a deadly explosion at an ammunition depot in the Czech Republic in 2014.