The Foreign Ministry threatened Romania with a tit-for-tat response on Wednesday after Bucharest ordered the expulsion of a Russian diplomat following Moscow's claims that it had caught a Romanian spy in Russia.
"We reserve the right to act in response," the Foreign Ministry said in a statement, without going into detail.
On Monday, the Federal Security Service said it detained Romanian diplomat Gabriel Grecu after he was caught "red-handed" trying to obtain military intelligence from a Russian counteragent.
Bucharest responded on Tuesday, ordering the first secretary of the Russian Embassy in the Romanian capital to leave the country within 48 hours.
The Russian Foreign Ministry statement said that while Moscow had evidence of Grecu's illegal activity, the Russian diplomat in Bucharest was "innocent."
"Linking these two issues means deliberately poisoning the atmosphere of Russian-Romanian relations," the ministry said.
Russia in February expressed concern over U.S. plans to install interceptor missiles in Romania in what Washington said was defense against potential threats from Iran.
The Romanian-Russian affair follows the U.S. arrests in June of 10 Russian undercover agents, who were later swapped for four Russians imprisoned on charges of passing secrets to the West.