Russia has condemned Ireland's decision to send home a Russian diplomat from its Dublin embassy over alleged involvement in passport forgery as an "unfriendly act" and vowed a response to the expulsion.

The Irish Foreign Ministry said on Tuesday it decided to expel the unnamed diplomat after an investigation found that Moscow's intelligence service had used Irish passports to produce fake documents supplied to a Russian spy ring based in the U.S.

A Russia-U.S. spy scandal broke out in late June when 11 Russian agents were arrested in the United States. The agents, who had worked under long-term cover, were freed in a swap deal between Russia and the U.S.

At least one of the spies allegedly used a fake Irish passport.

"This absolutely unfriendly act will not go without a corresponding reaction," Russian Deputy Foreign Minister Vladimir Titov said on Wednesday.

Ireland expelled an Israeli diplomat from its Dublin embassy in June over the alleged use of fake passports by the Israeli intelligence services in the assassination of a top Hamas official in Dubai.

In mid-December, Russia was embroiled in a diplomatic row with Britain after the British government expelled a Russian diplomat for espionage. Russia then requested the removal of a British diplomat.

MOSCOW, February 2 (RIA Novosti)