Nato has invited Montenegro to join the US-led military alliance, in its first expansion for six years.
The decision, described by Nato head Jens Stoltenberg as "historic", comes 16 years after the alliance bombed Montenegro during the Kosovo war, when it was still part of Yugoslavia.
Montenegro's accession would result in "retaliatory actions", said a spokesman for Russian President Vladimir Putin.
Accession talks are expected to take about a year to complete.
The invitation to Montenegro is Nato's first expansion into eastern Europe since Albania and Croatia joined in 2009.
The mountainous Adriatic state of 650,000 people has a small military with about 2,000 active members.
Nato diplomats say it sends a message to Russia that it cannot veto the alliance's expansion - but Russia has said it will retaliate.
"The continued eastward expansion of Nato and Nato's military infrastructure cannot but result in retaliatory actions from the east, i.e. from the Russian side, in terms of ensuring security and supporting the parity of interests," Mr Putin's spokesman Dmitry Peskov said.
Mr Peskov added that it was too early to specify what the retaliatory actions would be.