NATO has expelled eight members of Russia's mission to the military alliance, saying they were secretly working as intelligence officers.
The organisation said it had also cut the size of Moscow's team able to work at its headquarters.
"We can confirm that we have withdrawn the accreditation of eight members of the Russian mission to NATO, who were undeclared Russian intelligence officers," a NATO official said.
NATO had also reduced the number of positions that Russia could accredit people for at the organisation down to 10, the official said.
Relations between NATO and Russia have been increasingly strained since Moscow annexed Ukraine's Crimean Peninsula in 2014.
The two are at odds over Russia's nuclear missile development, aerial intrusions into NATO airspace and the buzzing of allied ships by fighter planes.
Watch: NATO test maritime strength in exercise off Scottish coast.
Official talks between them have been limited in recent years.
"NATO's policy towards Russia remains consistent. We have strengthened our deterrence and defence in response to Russia's aggressive actions, while at the same time we remain open for a meaningful dialogue," the official said.
The main forum for dialogue, the NATO-Russia Council, has 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.
Leonid Slutsky, chairman of the foreign affairs committee in the lower house of the Russian parliament, dismissed the accusations against the Russian diplomats as baseless and warned that NATO's move would further strain relations.
Mr Slutsky also told the Interfax news agency that Moscow could respond with "asymmetric" retaliatory measures, but he did not elaborate on what they might be.