US President Joe Biden has warned Vladimir Putin of dire consequences if he uses nuclear weapons in Ukraine.
Russian troops are on the back foot in the country and there are concerns tha t Putin will resort to trying more extreme methods of warfare in his campaign, such as tactical nuclear weapons and chemical attacks.
In a preview for an upcoming episode of 60 Minutes on CBS news, the president was asked what he would say to the Russian leader if he was considering unleashing a nuclear war.
Biden, explaining that the US would be forced to act if nukes were used, said: "Don't. Don't. Don't. You will change the face of war unlike anything since World War II.
"It'll be consequential [ Russia ] will become more of a pariah in the world than they ever have been. And depending on the extent of what they do will determine what response would occur."
The extent of what the response from the US side would be remains unclear, but there are further concerns it could lead to a dramatic escalation in the war.
The US has led the West in supplying arms to Ukraine to fight the invading Russian troops. The weapons include guided multiple launch rocket system (GMLRS) rockets, high mobility artillery rocket systems (HIMARS), Mi-17 helicopters, and stinger missiles among a number of others.
In total, the total US contribution to Ukraine in its fight for survival amounts to the equivalent of £13.6 billion.
More recently and as Ukraine's troops advance, Russia warned that the US would cross a "red line" if it decided to equip Kyiv with long-range missiles like army tactical missile systems (ATACMS).
The concern is that the use of ATACMS by Ukraine would mean the targeting of Russian territory, which may cause an escalation in the war.
Russian Foreign Ministry Spokeswoman Maria Zakharova said: "If Washington decides to supply longer-range missiles to Kyiv, then it will be crossing a red line, and will become a direct party to the conflict."
So far, Biden has ruled out providing Ukraine with this sort of assistance.
Biden said on Tuesday, September 13: "We’re not going to send to Ukraine rocket systems that strike into Russia."
Democratic Congressman Seth Moulton defended the stance and said to NBC: "I want the Ukrainians to have ATACMS as well but the Biden administration has solid rationale for not giving them these weapons at this moment and they briefed us on that this week.
"The broad criticism that the Biden administration could have acted more quickly in the past is fair, but they’re doing everything right. They’re thinking this through."
Russia's troops are facing a counteroffensive from Ukraine which has so far been successful in recovering areas like Kharkiv with key strategic towns like Izium and Kupiansk.
In Izium, a mass burial site containing 440 graves was found. There were signs of torture on the bodies, some with their hands tied behind their backs and marks indicating a "violent death".
