Russia has expelled two Canadian diplomats working for NATO's Moscow office in response to an "unfriendly act" by the military alliance.
Last week NATO expelled two Russian envoys from its headquarters in Brussels, reportedly due to spying.
NATO said in a statement that the Russian move was unjustified and "very unfortunate and counterproductive".
NATO on Tuesday began military exercises in Georgia, which are seen by Russia as a "provocation".
Last August Russian and Georgian forces fought a brief war over breakaway pro-Moscow Georgian territories.
Ottawa's ambassador to Russia, Ralph Lysyshyn, was told Moscow felt it had been forced into the move.
The Canadian diplomats being expelled are the director of the NATO information office in Moscow and her deputy.
