Before setting off for the UK for the funeral of Queen Elizabeth II, US President Joe Biden on Saturday urged Russian President Vladimir Putin not to use tactical nuclear or chemical weapons after setbacks in Ukraine.
Ukraine's military drove back Russian forces in a lightning rout in the north-east of the country this week, putting Mr Putin under pressure from nationalists at home to regain the initiative.
Mr Putin has issued a warning that Moscow would respond more forcefully if its troops were put under further pressure, raising concerns that he could resort to unconventional means, such as small nuclear or chemical weapons.
Asked by US broadcaster CBS what he would say to Mr Putin if he were considering using such weapons, Mr Biden said: “Don't. Don't. Don't. It would change the face of war unlike anything since the Second World War.”
Mr Biden said the US response would be “consequential” but declined to give detail. The Russians “would become more of a pariah in the world than they ever have been”, he said. “Depending on the extent of what they do will determine what response would occur.”
Russian government officials have dismissed western suggestions that Moscow would use tactical nuclear weapons in Ukraine, but it remains a worry for some in the West.
In a speech announcing the invasion of Ukraine on February 24, Mr Putin gave a veiled but unmistakable warning that if the West intervened in what he has called a “special military operation” he could use nuclear weapons in response.
“No matter who tries to stand in our way or … create threats for our country and our people, they must know that Russia will respond immediately, and the consequences will be such as you have never seen in your entire history,” he said, according to a Kremlin translation.
Other choices for the Russian leader could involve mobilising Russia's reserves, numbering around two million men, and pressuring Europe to strong-arm Ukraine into a truce with Russia, by freezing the region this winter by banning all energy exports.