Russia has reacted angrily to Ireland's expulsion of a Russian diplomat, warning that it will respond in kind.

Russia's Deputy Foreign Minister Vladimir Titov said in a statement carried Wednesday by Russian news agencies that Moscow sees Ireland's move as an "unfriendly act." He added that the Irish move "won't be left without a corresponding reaction."

The Irish government said it was expelling a Russian diplomat because Russian intelligence agents stole the identities of six Irish citizens to use as cover for spies operating in the United States.

Ireland opened the investigation after the U.S. Federal Bureau of Investigation smashed a Russian spy ring last June involving 10 men and women posing as American suburbanites.