Italy expelled two Russian officials on Wednesday after an Italian navy captain was caught red-handed by police selling secret documents to the Russians.
The Italian frigate captain was arrested on spying charges after officers tailing him saw him late Tuesday in Rome in a "clandestine meeting" with a Russian military officer, according to a police statement.
The two men met in a car park, according to media reports. Special operations police stopped them both but arrested only the navy officer, accusing him of passing on "confidential documents" in exchange for money.
The Russian, an embassy official, avoided custody thanks to diplomatic immunity, police said.
Italian Foreign Minister Luigi Di Maio summoned Russian ambassador Sergey Razov on Wednesday morning to lodge a formal protest.
The diplomat was notified of "the immediate expulsion of the two Russian officials involved in this very serious affair," Di Maio said.
Eleonora Tafuro, a Russia expert at the ISPI think tank in Milan, said such incidents in Italy were rare.
"It is very serious ... and really takes us back to the Cold War period," she told AFP.
The ANSA news agency, quoting investigative sources, named the navy officer as Walter Biot, and said he worked in the military policy unit within the office of the Chief of the Defense Staff.
That unit handled "all confidential and classified documents," including NATO dossiers, the Corriere della Sera newspaper said.
It also reported that Biot received 5,000 euros ($5,860) in cash from his Russian contact.
A NATO official referred queries to Italian authorities, saying: "We don't comment on intelligence issues."
The navy captain took pictures of secret documents on his computer screen and passed the files onto a pen drive which police confiscated, according to the AGI news agency.
In London, Britain's Foreign Secretary Dominic Raab expressed "solidarity" with Italy and condemned "Russia's malign and destabilizing activity that is designed to undermine our NATO ally."
Police said the suspected spy was discovered after long investigations led by Italy's domestic intelligence agency AISI, with support from the Chief of the Defense Staff.
Moscow is currently embroiled in a series of rows with the West, most recently over the jailing of Kremlin critic Alexei Navalny, a move that triggered EU sanctions against senior Russian officials.
But Italy is one of the countries within the European Union and NATO with the warmest relations with Russia. Former prime minister Silvio Berlusconi is a friend of Russian President Vladimir Putin.
Kremlin spokesman Dmitry Peskov said he hoped that "the very positive and constructive nature of Russian-Italian relations will continue and will be preserved."
The Russian Embassy in Rome confirmed that a member of the office of its military attache was stopped by police on Tuesday, but said it was "inappropriate to comment" in detail.
In a later statement, the embassy expressed "regret" for the expulsion of two members of its military attache office, but made no mention of possible retaliatory moves.
Tafuro suggested that Moscow might play down the affair to avoid antagonizing an ally while it is trying to get the bloc to approve the use of the Russian Sputnik V coronavirus vaccine.
"Russia is pushing hard [for this]" and "Italy has often spoken up for this Russian request," so ruining relations with Rome "could be a counterproductive move," the analyst said.
Bulgaria, an EU and NATO member like Italy, expelled two Russian diplomats last week after six people were arrested, including several defense ministry officials, on suspicion of spying for Russia.
Also last week, the Kremlin issued a statement in which Putin bemoaned "the unsatisfactory state of Russia-EU ties," which he blamed on the "unconstructive, often confrontational policies of our partners."
Earlier this month, relations between Moscow and Washington sank to a new low after U.S. President Joe Biden called the Russian president a "killer," leading Putin to say: "It takes one to know one."