Russia on Wednesday said it would be forced to react to NATO expansion after the US-led alliance invited Montenegro to become its 29th member.
The invitation to the small Balkan country comes with the West at loggerheads with Moscow over a host of issues, as NATO has responded to the Ukraine crisis with a military upgrade to reassure nervous ex-Soviet states they need not fear a more assertive Russia.
"On all different levels Moscow has always noted that the continuing expansion of NATO, the military infrastructure of NATO, to the east of course cannot but lead to reciprocal actions from the east, that is from the Russian side, in the interests of providing security and supporting the priority of our interests," Kremlin spokesman Dmitry Peskov told journalists.
Announcing the move at a meeting of NATO foreign ministers in Brussels, alliance chief Jens Stoltenberg insisted the "historic" invitation to Montenegro was no one else's business and "not directed at anyone."
"It is extremely important to underline once again that every nation has the right to decide its own path, its own security arrangements," Stoltenberg said. "No one else has the right to interfere in that decision."
Russian President Vladimir Putin has bitterly complained of what he sees as NATO encroachment and last week, Moscow condemned the expected Montenegro invitation as a "serious blow by the euro-Atlantic bloc."
"This sort of initiative has the real potential to bring about confrontation. It will not promote peace and stability in the Balkans nor in Europe in general," the Russian foreign ministry said.
"It will only further complicate relations between Russia and NATO."
The tiny Balkans country of just over 600,000 people won its independence in 2006 following the bloody break-up of what was Yugoslavia.
It had been part of a federation with Serbia, a long-time Russian ally which Moscow has always regarded to be part of the same Slav family.