The White House is concerned about what it says was Russian interference in NATO-aspirant Montenegro's parliamentary elections last year.
"We are very concerned about Russian interference in the October elections in Montenegro, including credible reports of Russian support for an attempted election day attack on the government," a senior White House official told reporters on April 12.
The comments came a day after President Donald Trump approved Montenegro's NATO accession, and shortly before he met with NATO Secretary-General Jens Stoltenberg at the White House.
A special prosecutor in the former Yugoslav republic said a group of "Russian nationalists" had planned to assassinate then-Prime Minister Milo Djukanovic to get an opposition party into power.
Moscow has denied involvement in any plot, and Montenegrin opposition parties have said the plot was fabricated.
Trump signed ratification papers on April 11 accepting Montenegro into the NATO alliance, saying Montenegro's NATO membership will increase stability and security in the Western Balkans.
"After meeting the rigorous standards to join the alliance Montenegro's accession will make clear that no third country has a veto over a country’s sovereign decision to join NATO. So the door to membership in the Euro-Atlantic community of nations remains open," the White House official said.