Russia will react to a Nato military build-up on its borders, President Vladimir Putin has warned.
Vladimir Putin is warning NATO against 'muscle-flexing'
He said the alliance was violating earlier agreements, and confirmed that Russia would pull out of a key arms control accord with Nato countries.
Moscow is strongly opposed to plans by the alliance to set up missile defence sites in Poland and the Czech Republic.
Nato says the system is not aimed at Russia, but is necessary to guard against attacks from "rogue states".
Mr Putin told senior Russian generals on Tuesday: "In violation of previous agreements, certain member countries of the Nato alliance are increasing their resources next to our borders.
"Russia cannot remain indifferent to the clear muscle-flexing."
Mr Putin said Russia's decision to suspend the Conventional Forces in Europe (CFE) Treaty - which limits the deployment of tanks, aircraft and other heavy conventional weapons - was part of Moscow's response.
Cornerstone of European security
Limits amount of key military equipment in designated area
Negotiated by Nato and ex-Warsaw Pact states
Signed in 1990
Came into force in 1992
Nato never ratified revised 1999 version
Mr Putin appeared to leave the door open to Russia resuming participation in the treaty, but said this would be conditional on Nato ratifying a new version of the accord.
"We won't observe any obligations unilaterally", he said
The CFE treaty was signed by Western and former Warsaw Pact states in 1990, but was revised in 1999 to take account of former Soviet satellites joining Nato.
However the alliance - unlike Russia - has refused to approve the updates until Moscow pulls its remaining troops out of Georgia and Moldova.
Defence is one of many areas where relations between Moscow and the West have deteriorated as Russia increasingly seeks to assert itself on the international stage.