Montenegro is set to become NATO's 29th member following the Senate's overwhelming ratification yesterday.
Russia, though, strongly opposes the Western military alliance's expansion in the Balkans. Russia has been accused by Montenegro of trying to stage an election day coup in October to try to keep the country out of the alliance.
Montenegro Prime Minister Dusko Markovic said that Montenegro never doubted US support for the country.
He said Montenegro's membership in NATO will be its biggest foreign policy success since it gained independence and split from much-bigger Serbia in a 2006 referendum.
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The pro-Russian opposition parties in Montenegro reacted by saying they are demanding a referendum on NATO entry.
The NOVA party said in a statement that the membership would represent "a brutal occupation" of Montenegro, adding that the alliance's soldiers would be treated as "occupiers."
US Senators voted 97-2 yesterday to admit Montenegro, with only Republican Sens. Rand Paul of Kentucky and Mike Lee of Utah voting "no." Paul had suggested that adding Montenegro could lead to heightened tension with Moscow, possibly even war.
Montenegro's membership gives NATO a contiguous border along the Adriatic coast.
Ratification from Spain and the Netherlands is still pending ahead of a NATO summit in May.