An Italian navy officer has been arrested after being caught in the act of handing secret documents to a Russian military official, police say.
The men were stopped by military police on suspicion of serious crimes relating to espionage and state security.
The Russian, who reportedly worked at the Russian embassy in Italy, is expected to be expelled.
The foreign ministry in Rome summoned Russia's ambassador Sergey Razov and then expelled two Russian officials.
Italian Foreign Minister Luigi Di Maio said the pair were "involved in this extremely serious affair". The Italian officer was identified as a navy frigate captain working in the office of Chief of the Defence Staff.
The Kremlin said it hoped the "very positive and constructive nature of Russian-Italian relations will continue" but Russia's foreign ministry is expected to respond to the expulsions.
What happened in Rome
The carabinieri del Ros special operations group swooped on the men in Rome on Tuesday evening "during a clandestine meeting between the two," a police statement said.
They were "caught immediately after the transfer of a document by the Italian officer in exchange for a sum of money", it added.
Italian media reported that the exchange had taken place in a car park and involved €5,000 (£4,250; $5,900) in cash.
While the navy captain was arrested, Italian police said the Russian officer's diplomatic status was currently being assessed.
Italy's Corriere website reports that papers seized in the naval officer's flat suggest he may have passed on Nato secrets, thereby placing other countries' national security at risk. According to the Ansa news agency, the documents involved military communications.
The Russian embassy confirmed the involvement of a military attaché official but said any further comment was inappropriate.
Kremlin spokesman Dmitry Peskov said "we don't have information about the reasons for or the circumstances of this detention". The foreign ministry in Moscow told Ansa that it regretted the expulsions of two embassy staff and would make a further announcement on possible steps in response.
Source: BBC
