Italian police said earlier they had arrested an Italian navy captain and a Russian diplomat on suspicion of spying after the two were caught swapping documents for money at a clandestine meeting.
Di Maio wrote on Facebook that he had protested firmly to the Russian ambassador and informed him of the "immediate expulsion of two Russian officials involved in this extremely serious matter."
As we reported earlier, Bulgarian prosecutors said they had charged six Bulgarians, including several senior officials in the defense ministry and in military intelligence, with espionage.
At least five former or current members of the Bulgarian military were arrested, said Siyka Mileva, spokeswoman for the general prosecutor.
The investigation was of "special significance" for the national security of Bulgaria, the EU, NATO and the United States, general prosecutor Ivan Geshev told the press.
Among those arrested was a former military attache, currently in charge of classified information at Bulgaria's parliament.
Another is a former high-ranking military intelligence officer in the Defense Ministry who was accused of being the head of the spy network, Mileva said.
The officer in question was allegedly trained by the Russian military intelligence and his task was to recruit an "illegal network of agents" made up of people who had access to classified information about Bulgaria, NATO and the European Union.