Moscow on Monday announced it was expelling an Italian diplomat in response to the "unfriendly" removal of two Russian officials from Italy in March over spying allegations. 

The foreign ministry said it had summoned Italy's ambassador to Russia and that he was informed that Russia was "declaring 'persona-non-grata'" Rome's naval attache in Moscow, who would have 24 hours to leave the country.

Italy condemned Russia's move as "unfounded and unjust" and expressed its "deep regret" at the decision.

"We consider the decision unfounded and unjust because it is in retaliation for a legitimate measure taken by the Italian authorities in defense of their own security," said the country's Foreign Ministry.

Italy expelled two Russian diplomats after police said they had caught an Italian navy captain passing secret documents to a Russian military official in return for money.

Also on Monday, Romania's Foreign Ministry said it will expel a Russian diplomat, the latest European country to do so in solidarity with the Czech Republic, which is involved in a spat with Moscow. 

Alexei Grichayev, the deputy military attache at the Russian embassy in Bucharest, will be declared "persona non grata" in view of his activities and actions contrary to the Convention of Vienna on diplomatic relations, the ministry said in a statement.

(With input from Reuters)