Montenegro has protested the "inappropriate" treatment of a lawmaker who was prevented from changing airplanes at a Moscow airport.
Miodrag Vukovic, a prominent deputy from the ruling Democratic Party of Socialists (DPS), said he was kept overnight in a transit zone at Domodedovo International Airport while on his way to Belarus. He returned to Montenegro on May 29.
In a statement on May 29, the Montenegrin Foreign Ministry said the incident "represents a breach of basic international rules and diplomatic practices."
Russian Foreign Ministry spokeswoman Maria Zakharova said the Kremlin imposed a ban on Vukovic because Montenegro had joined Western sanctions against Moscow over its aggression in Ukraine.
"It's not a secret that Montenegro joined the European Union's anti-Russian sanctions, including against [Russian] individuals," Zakharova said on the Foreign Ministry's website.
"We have always said that we reserve the right to take retaliatory measures on the basis of reciprocity, as is customary in diplomacy," she said. "We will provide a relevant explanation to the Montenegrin side."
Vukovic claims the incident was politically motivated.
Montenegro was previously a close ally of Russia but is set to join NATO in June.
That decision has drawn derision and outrage from NATO critics in the Balkan country and Russia, which has threatened economic and political retaliation.
Montenegro claims Russia was behind a foiled coup attempt in October 2016, which Moscow denies.