The United States has been sending secret warnings to the Kremlin for months about the grave consequences of using nuclear weapons in Ukraine, it has emerged.
Washington has not spelled out to Moscow exactly what its response would be, maintaining a position of “strategic ambiguity”.
The US has kept the warnings vague in the hope of inducing panic and uncertainty in the Kremlin.
Despite the warnings Vladimir Putin, in a televised national address on Wednesday, implicitly threatened a nuclear strike.
It was unclear how, or by whom, the US messages were delivered, or whether any further communications had been sent since Mr Putin’s speech.
US intelligence officials have indicated there is currently no sign Russia is moving its nuclear weapons to get ready for a strike.
There has been concern Russia could opt to use a small tactical nuclear weapon, sometimes referred to as a “battlefield nuke”.
Analysts believe Mr Putin could do so if he believed his own position is under threat.
His intention would be to “escalate to de-escalate”, ending the war having secured some gains, but causing devastation in the process.