President Donald Trump has signed the U.S. instrument of ratification for Montenegro's accession to NATO, the White House says.
A statement said Trump signed off on Montenegro's accession to the Western military alliance on April 11, congratulating the Montenegrin people for "their resilience and their demonstrated commitment to NATO's democratic values."
Last month, the U.S. Senate overwhelmingly backed the expansion of NATO to allow Montenegro to become the alliance's 29th member.
Russia, which strongly opposes NATO's expansion, has described Montenegro's upcoming accession as a "provocation."
"The door to membership in the Euro-Atlantic community of nations remains open," the White House said, adding that countries in the Western Balkans were "free to choose their own future and select their own partners without outside interference or intimidation."