Russia’s nuclear forces have been put on high alert to a "special mode of combat duty" by President Vladimir Putin.
The Kremlin said the move followed comments by UK Foreign Secretary Liz Truss and others.
Kremlin spokesman Dmitry Peskov said "unacceptable" remarks were made about possible "clashes" between Nato and Moscow over Russia’s attack on Ukraine.
The US immediately condemned the decision, calling it an "unacceptable escalation".
The BBC’s Ros Atkins looks at the history of nuclear tensions and deals between the US and Russia.
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