Russia's President Vladimir Putin says he is moving Russia's nuclear deterrent to "special alert".
He says it's a response to Nato "aggression".
Speaking to top military officials, including Defence Minister Sergei Shoigu, Putin said Western nations had taken "unfriendly actions" towards Russia and imposed "illegitimate sanctions".
The move makes it easier to launch weapons - but the BBC's security correspondent Gordon Corera said it was a way for Russia to send a warning to Nato, rather than signalling an intent to use them.
