Moscow expelled a Bulgarian diplomat on Thursday in response to Bulgaria´s expulsion of an alleged Russian spy.
The reciprocal move saw the foreign ministry in Moscow calling the Bulgarian ambassador an undesirable figure after Sofia expelled the first secretary of the Russian embassy for alleged spying.
Bulgaria, Russia´s closest ally during the communist era, also refused to grant a visa to a Russian diplomat for the same reason.
The outbreak of the diplomatic row comes as Bulgaria is building the Turk Stream gas pipeline through the European state.
Russian President Vladimir Putin on Wednesday accused Bulgaria of deliberately delaying the construction of a gas pipeline from the Bulgarian-Turkish border to Serbia.
Bulgarian authorities expelled a Russian diplomat earlier on suspicion of spying.
The expelled Russian diplomat is believed to have had regular meetings since September last year with a senior Bulgarian official with access to classified information about Bulgaria, the European Union and NATO.
Bulgarian prosecutors said the purpose of the meetings was to obtain confidential information.
Bulgarian authorities temporarily arrested eight citizens in September as part of an investigation into a spy case. Among those arrested was Nikolai Malinov, head of the Russian-Bulgarian Friendship Movement, who rejected the charges.
