Rome: Italy on Wednesday ordered two Russian Embassy officials to oust and arrested an Italian Navy captain on spying charges.

According to an international news agency, police caught an Italian navy captain who allegedly handed over classified documents to the Russians in exchange for money.

The Italian Foreign Minister Luigi Di Maio summoned Russian Ambassador Sergey Razov, and protested the "grave incident" after a sting operation on the outskirts of Rome.

The diplomats were notified of "the immediate expulsion of the two Russian officials involved in this very serious affair", Di Maio said.

A frigate captain was also arrested on spying charges after a "clandestine meeting" with the Russian late on Tuesday in Rome, according to a police statement.

Special operations police stopped both men but only the Italian was arrested. He is accused of passing on "confidential documents" in exchange for money.

The Russian embassy official avoided custody thanks to diplomatic immunity, police said.

According to a local Italian newspaper, 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.

Corriere della Sera newspaper said the captain was paid 5,000 euros ($5,860) in cash by the Russians.

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. 

The latest episode is termed as the "most serious" incident with Russia since the end of the Cold War, recalling a 1989 incident when Russian and Bulgarian spies were discovered in Italy.

The Russian Embassy in Rome confirmed the detention of the diplomats who were part of the military attache's office but wouldn't comment on the incident.

"In any case, we hope that it wouldn't affect bilateral ties," it said in a statement.

Speaking in a conference call with reporters, Russian President Vladimir Putin's spokesman, Dmitry Peskov, said the Kremlin was unaware of details of the incident but noted that "we hope that the quite the positive and constructive character of the Russian-Italian relations will be preserved."

The Russian Foreign Ministry voiced regret about the diplomats' expulsion in a concise statement carried by Russian news agencies.