Russia said on Monday it was expelling two Bulgarian diplomats in a tit-for-tat move after two Russians were ordered out of the EU country on accusations of spying for the military.

The Foreign Ministry said in a statement that Bulgarian ambassador Atanas Krystin was summoned to be informed that the two Moscow-based diplomats had been declared “personae non gratae.”

“This step is a tit-for-tat ‘retaliatory’ response to an unwarranted decision of the Bulgarian authorities to expel from the country in September two representatives of Russia’s trade mission in Sofia,” the ministry said.  

In September, Bulgaria said it had expelled the two Russians on accusations of spying on the military.

Moscow at the time said it viewed the decision as a “deliberate attempt to harm constructive Russia-Bulgaria cooperation.”

The Bulgarian public prosecutor alleged the Russians had been seeking out classified information on military modernization plans since 2016.

The controversy is likely to worsen traditionally warm but now fraying relations between the two countries after last year saw the ejection of three other Russian diplomats over allegations of obtaining classified information.

A former Warsaw Pact ally of Moscow, Bulgaria’s military retains military hardware dating from that era and is now modernising to conform with NATO guidelines after joining the Western alliance in 2004.