The US Senate voted overwhelmingly to advance the approval of Montenegro as the newest member of NATO, in what supporters of the alliance's expansion argue would send a stern message to Russia.
The procedural step, which advanced on a 97-2 vote, sets up a final approval in the chamber in the coming days.
President Donald Trump's administration has encouraged lawmakers to back the small Balkan nation's bid.
"It is strongly in the interests of the United States that Montenegro's membership in NATO be ratified," Secretary of State Rex Tillerson told Senate leaders in a March 7 letter.
To date, 25 of NATO's 28 members have ratified Montenegro's accession, a country of 620,000 people seen as a geostrategic ally.
The North Atlantic Treaty Organization holds its summit on May 25 in Brussels, where Trump will use the opportunity to reaffirm Washington's strong commitment to the alliance, according to the White House.
The Kremlin is opposed to Montenegro's accession, calling it a "provocation" that would reinforce the pro-Western military alliance's presence in the Balkans.
The US vote comes days after a Montenegrin special prosecutor accused "Russian state bodies" of involvement in an alleged coup plot during Montenegro's election last October. Moscow branded the accusation "absurd." Russia also stands accused of interfering in the US presidential election last year, when US intelligence agencies say it leaked hacked emails that damaged Democrat Hillary Clinton's campaign.
Senator John McCain, among the move's strongest backers, framed Montenegro's accession as nothing less than a "test" of resolve against Russian President Vladimir Putin.
"He attempted a coup" to overthrow the freely-elected Montenegro government," McCain told the Senate Monday.