NATO chief Jens Stoltenberg said Thursday the expulsion of eight members of the Russian Mission to the alliance was not the result of any one particular event, but due to an “increase in malign activity for some time.”

NATO officials confirmed Wednesday they were expelling eight Russian diplomats and planned to reduce the size of Russia’s observer mission to the Western alliance from 20 to 10. The moves are in response to alleged Russian espionage activities in Europe, which reportedly included killings, attempted assassinations and explosions.

Speaking to reporters following Thursday’s meeting of NATO’s national security advisers at the alliance headquarters in Brussels, Stoltenberg said they need to be vigilant and act when members of the Russian delegation to NATO “conduct activities which are not in line with their accreditation and therefore, the accreditation is withdrawn.”

Relations between NATO and Russia have been strained since Moscow’s 2014 annexation of Crimea and its fomenting and ongoing involvement in a separatist conflict in eastern Ukraine.

Stoltenberg acknowledged the relationship between NATO and Russia is at its lowest point since the end of the Cold War, and he blamed it on Russia’s behavior.

“We have seen their aggressive actions not least against Ukraine but also the significant military buildup,” he said.

Although Russia is not a member of the U.S.-led alliance, it has long had an observer mission to NATO as part of a two-decade-old NATO-Russia Council, the main forum for dialogue between the alliance and Moscow. The forum has been mostly inactive since 2014.

Stoltenberg said the NATO has invited Russia for some time to take part in new NATO-Russia Council talks, typically convened to promote cooperation between the alliance and the nation, but Russia has not accepted.

But Stoltenberg said NATO is “ready to meet, because we believe that to sit down to talk is always important.”

Some information for this report came from the Associated Press, Reuters, and Agence France-Presse.

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