Nato ordered the expulsion of the two Russian diplomats on Wednesday
Russia has confirmed Nato has expelled two of its diplomats from Brussels, reportedly in retaliation for a spy scandal involving an Estonian official.
In a statement, the Russian foreign ministry described the expulsions as "outrageous" and a "crude provocation".
Nato diplomats claimed the Russians were undercover intelligence agents.
The official, Herman Simm, was jailed for 12 years in February by an Estonian court for passing Nato defence and diplomatic secrets to Moscow.
The court where the former head of Estonia's national security system was tried did not reveal which country he spied for, but investigators said Mr Simm passed nearly 3,000 documents to Russia.
They said he received 1.3m kroons (£74,000; $110,000) for the data. The Kremlin denied any involvement.
Nato made no comment at the time, but the case, Estonia's biggest spy scandal since the Cold War, was seen as an embarrassment for the former Soviet state, which joined the alliance in 2004.
On Thursday, Russia's foreign ministry confirmed that Nato had expelled two of its diplomats, but said the move was unjustified.
"A crude provocation has been made in relation to two employees of Russia's permanent mission to Nato on an absolutely trumped up pretext without any clear explanation," it said.
Russia's ambassador to Nato, Dmitry Rogozin, said he had been informed by the alliance's secretary general, Jaap de Hoop Scheffer, on Wednesday that it had decided to withdraw the accreditation from two of his staff.
"I was told the following: 'Nato is outraged by Russia's spying activities against Nato and alliance member states'," he said.
Mr Rogozin identified the diplomats as Viktor Kochukov, a senior counsellor at the Russian mission and the head of its political section, and Vasiliy Chizhov, a lower-level attache and the son of Vladimir Chizhov, Russia's envoy to the European Union.
"There will be a response to such steps, which Nato will learn about shortly," he warned, adding that "someone in the West strongly dislikes how relations between Russia and individual countries of Western Europe are developing".
A Nato spokesman said he could not confirm the story because he could not comment on intelligence matters.
The move came as Nato held its first formal talks with Russian representatives since last summer's war between Russia and Georgia over the breakaway region of South Ossetia.
Investigators said Simm, who used to be the chief of Estonia's police service, had been working for the Russian Foreign Intelligence Service (SVR) since 1995, when he joined the Estonian defence ministry.
Simm, who was convicted of treason, was ordered to pay 20.2m kroons (£1.15m; $1.71m) to the Estonian defence ministry for the cost of new security systems.