Montenegro ordered the expulsion of six Russian diplomats Thursday amid a major investigation into alleged spying, prompting Moscow to shutter its consulate in the Balkan nation.

Axar.az reports citing foreign media that the six, who worked for the Russian embassy in Podgorica, were declared persona non grata due to “activities that are contrary to the Vienna Convention on Diplomatic Relations”, the foreign ministry tweeted.

Moscow retaliated by closing its consulate in Montenegro “indefinitely”, Russia’s embassy in the small Balkan nation said in a statement.

“In relation with hostile actions by Montenegrin authorities towards the Russian embassy in Montenegro, working of consular department in Podgorica is closed indefinitely,” Russia’s embassy in Podgorica said.