Montenegro expelled six Russian diplomats on Thursday, announcing an investigation into people suspected of spying for Russia.
Moscow retaliated by closing its consulate in the tiny Balkan nation, saying its decision was because of hostile actions by authorities towards the Russian embassy in Montenegro.
Russia's embassy in Podgorica, Montenegro's capital, said it was shutting down "indefinitely."
Montenegro's Foreign Ministry said the six diplomats, all of whom worked for the Russian embassy in Podgorica, were ordered to leave "due to activities in contravention of the Vienna Convention on Diplomatic Relations and mutual respect and appreciation of the two countries."
How did tensions escalate?
Montenegro launched an investigation into an unspecified number of people earlier on Thursday, state prosecutors announced.
Vukas Radonjic, a spokesperson for Montenegro's State Prosecutor's Office, said the individuals were suspected of "creating a criminal organization and espionage."
Local media reported that people were suspected of spying for Russia and up to 30 Russian nationals with temporary residence permits were detained. Authorities detained two Montenegrin citizens as well.
Montenegro's outgoing Prime Minister Dritan Abazovic said the operation was undertaken along with Montenegro's international partners to "preserve national interests." However, he didn't mention Russia by name.
Montenegro joined NATO in 2017, despite strong protests by Russia. The Balkan nation, a historically Russian ally, relied on Russian and Ukrainian tourists for revenues until Russia invaded Ukraine in February.
Montenegro, a candidate country to be a member of the European Union, adopted EU sanctions against Russia in April.