Russia's foreign ministry said the United States's approval of Montenegro's bid to join NATO was a "deeply mistaken" move that creates divisions in Europe.
It "reflects the logic of confrontation on the European continent and creates new dividing lines," the ministry said on April 13.
U.S. President Donald Trump signed the ratification documents for Montenegro's entry into NATO on April 11.
"We consider the course towards including Montenegro in NATO is deeply erroneous, goes fundamentally against the interests of people in this country, and harms stability in the Balkans and in Europe as a whole," the Russian ministry said.
The ministry said the accession was forced upon Montenegro through what it called "puppet NGOs" that work to "create the illusion of public support for the one-sided policy of the Montenegrin authorities."
The population of Montenegro, on the Adriatic Sea, is mostly Orthodox Slavs and Moscow has long considered it to be within its sphere of influence.