Russia has declared assistant military attache Mitko Borisov persona non grata and given him 72 hours to leave the country, the Russian Foreign Ministry said in a statement.
"The step has been taken in response to the unjustified decision by Bulgaria to expel Russia's military attache in December", the statement read.
Bulgaria previously expelled and declared persona non grata a Russian military attache after its Foreign Ministry said it had been informed by prosecutors that the person in question was suspected of collecting classified information.
The Russian Foreign Ministry called the move groundless and vowed to respond to the expulsion, noting the move was not conducive to a dialogue between the two countries on military issues or ensuring stability in the Black Sea region.