Russian President Vladimir Putin has suspended involvement in one of the key post-Cold War arms control treaties.
In a statement, the Kremlin said the choice was due to "extraordinary circumstances" affecting security.
Russia has been angered by US plans to base parts of a missile defence system in Poland and the Czech Republic.
The Conventional Forces in Europe Treaty limits the number of heavy weapons deployed between the Atlantic Ocean and the Urals mountains.
'Cornerstone'
A Nato spokesperson said that the alliance's secretary general would "very much regret the decision" if confirmed.
"The allies consider this treaty to be an important cornerstone of European security," he said.
In a separate interview, Nato spokesperson James Appathurai told the BBC the move was "a disappointing step in the wrong direction".
The CFE agreement, one of the key Cold War European security accords, was signed eight years after the Warsaw Pact was dissolved.
It sets limits for the number of tanks, heavy artillery and combat aircraft, as well as troops, deployed in the region.
The BBC's Europe editor, Mike Saunders, says that the US announcement of its plans for a missile defence shield within Europe was the last straw for Russia.
Mr Putin saw the plans as further unbalancing a treaty that already limits the conventional arms Russia can deploy on its own territory.
The Kremlin maintains that the 1990 treaty is outdated and restricts its ability to move troops around its own territory.
Russia ratified the 1999 revised version, but Nato has not done so.
Nato is first demanding the withdrawal of Russian forces from two breakaway regions with Russia-speaking majorities - Abkhazia in Georgia and Trans-Dniester in Moldova.
(BBC News)
