Russia has formally suspended its participation in a key arms control agreement dating from the Cold War.
The treaty limits numbers of troops and weapons in Europe
The Conventional Forces in Europe (CFE) treaty sets limits on troops and weaponry across Europe.
The suspension means Russia can move troops without notifying Nato. The bloc voiced "deep regret" over the move.
Russia is unhappy with Nato expansion and US plans for missile defences in central Europe and says the treaty no longer serves its interests.
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.
'No troop build-up'
Russia suspended its participation in the CFE at midnight on Wednesday (2100 GMT Tuesday).
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
The foreign ministry in Moscow said Russia had stopped exchanging information stipulated by the treaty and had stopped receiving foreign inspectors.
"At the same time we do not have plans in the current circumstances for a massive build-up or concentration of forces of the borders with neighbours," it said in a statement.
Nato expressed it "deep regret" over Moscow's move, but stressed that the military alliance would not take any retaliatory action.
"Nato allies... want to resolve the current impasse and preserve the benefits of this landmark treaty," a Nato statement said.
"They have therefore chosen not to respond in kind at this stage," it said.
The Organisation for Security and Cooperation in Europe (OSCE), the continent's main security watchdog, criticised Moscow for undermining "a cornerstone" of efforts to reduce tensions in Europe.
"The suspension... is not good news," Spanish Foreign Minister Miguel Angel Moratinos, whose country holds the rotating chairmanship in the OSCE, said in a statement.
In practical terms the Russian move means that a whole range of inspections and transparency measures intended to bolster mutual military trust will be suspended, the BBC's diplomatic correspondent Jonathan Marcus says.
That, arms control experts argue, will be a loss.