U.S. President Joe Biden said the current risk of Russia deploying a tactical nuclear weapon is greater than it has been at any time since the Cuban missile crisis, an assessment that overnight raised fears of a nuclear war to new heights.
Putin has repeatedly warned the West that any attack on Russia could provoke a nuclear response.
It comes as Ukrainian forces reclaim dozens of settlements in the south of Ukraine, adding to a growing list of military setbacks for the Kremlin.
Ukrainian President Volodymyr Zelenskyy said the country's forces have recaptured 500 square kilometers (193 square miles) in the Kherson region alone since the start of October.
Britain's Defense Ministry says more than half of Ukraine's fielded tank fleet could consist of captured Russian vehicles.