Romania has condemned Russia for arresting and expelling one of its diplomats, and said a Russian envoy would be deported in return.
The Romanian foreign ministry said it would "declare a diplomat in the [Russian] embassy - with an equivalent ranking - as persona non grata".
On Monday Russia's FSB security agency said it had caught diplomat Gabriel Grecu "red-handed" while spying.
It said he had 48 hours to leave the country.
The Romanian foreign ministry condemned what it called a "severe breach by Russian authorities of the 1961 Vienna Convention regarding diplomatic relations".
The FSB had said that Mr Grecu was detained "during an attempt to receive secret information of a military nature from a Russian citizen".
In a further statement carried by the Interfax news agency on Tuesday, it said: "In addition to open political information, Grecu ordered the Russian citizen to pass over documents containing military secrets to him.
"After that, citizen M, realising that the Romanian spy was inducing him to commit treason, voluntarily reported this to the FSB and said he was ready to co-operate with the security agencies to prevent the diplomat's unlawful activities."
The statement said the documents, if passed to foreign special services, would have resulted in significant damage to Russia's security interests, and therefore "it was decided to stop Grecu's spying activity".