BBC: 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". Russia pulls out of key arms pact (more)
Mark Alexander
