Italy summoned the Russian ambassador in protest Wednesday after an Italian navy captain was arrested for allegedly passing confidential documents to a Russian military official.
The Italian, a middle-ranking officer serving as a frigate captain, was caught red-handed after a "clandestine meeting" with the Russian late Tuesday in Rome, according to a police statement.
Special operations Carabinieri police swooped in and stopped both men, but only the Italian was arrested. He is accused of passing on the documents for money.
The Russian, an embassy official, avoided custody thanks to his diplomatic immunity, the statement said.
According to La Repubblica daily, the navy captain worked at the office of Chief of the Defence Staff and had access "to a wide range of documents" concerning both Italian defence and NATO activities.
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 Defence Staff.
La Repubblica said it was the "most serious" incident with Russia since the end of the Cold War, recalling a 1989 precedent when Russian and Bulgarian spies were discovered in Italy.
Reacting to the arrest, Italy's foreign affairs ministry said it had summoned the Russian ambassador to Rome, Sergey Razov, for urgent talks with the ministry's top civil servant.
- Heightened tensions with West -
Within the European Union and NATO, Italy is one of the countries with the warmest relations with Russia. Its former premier Silvio Berlusconi is a personal friend of Russian President Vladimir Putin.
The Russian embassy in Rome confirmed that their military attache was stopped by police on Tuesday, but said it was "inappropriate to comment" in detail.
"In any case, we hope that what happened will not affect the bilateral relationship between Russia and Italy," the embassy said, adding that Razov was on the way to the foreign ministry.
The affair comes against the backdrop of heightened tensions between Moscow and the West, most recently over the jailing of Kremlin critic Alexei Navalny, a move which triggered EU sanctions against senior Russian officials.
Bulgaria, an EU and NATO member like Italy, expelled two Russian diplomats last week after six people were arrested in the country, including several defence 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 US President Joe Biden called the Russian president a "killer," leading Putin to say, "it takes one to know one."