NATO on Wednesday expelled eight members of Russia's mission to the military alliance on the grounds that they were in fact working secretly as spies.

"We can confirm that we have withdrawn the accreditation of eight members of the Russian Mission to NATO, who were undeclared Russian intelligence officers,'' a NATO official said, speaking under condition of anonymity.  

The officials said NATO had also reduced the size of the team Moscow that is permitted to have working at the military alliance's headquarters to 10, though no clear reason was given for that decision.

Retaliation from Moscow?

Russian news agency Interfax cited senior lawmaker Leonid Slutsky, head of the lower house of parliament's international affairs committee, as saying Moscow would retaliate, though not necessarily in kind.

The moves come as relations between NATO and Moscow remain strained, notably over Russia's annexation of the Crimean Peninsula from Ukraine in 2014 and its support of a rebellion in eastern Ukraine.

The NATO official said the alliance had stepped up "deterrence and defense" as a response to "Russia's aggressive actions" but was still "open for a meaningful dialogue" with Moscow.

A string of countries in or close to NATO, particularly in eastern Europe, have asked Russian diplomats to leave on similar grounds of alleged espionage activity in recent months. Russia often responded in kind by demanding that diplomats leave its territory.

tj/msh (AP, Reuters, dpa)