Kyiv, Ukraine - President Joe Biden has warned Russia against using nuclear weapons in its war on Ukraine.

"They'll become even more of a pariah in the world than they ever have been," Biden told CBS in an interview.

Asked how the US government would react in such a case, Biden said, "You think I would tell you if I knew exactly what it would be? Of course I'm not going to tell you."

However, he made it clear that there would be serious consequences, saying it would "change the face of war, not like anything since World War II."

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The full interview is due to be broadcast on 60 Minutes on Sunday.

Meanwhile, in Kyiv, Ukrainian President Volodymyr Zelensky has accused Russian forces previously occupying much of Ukraine's Kharkiv region of employing the widespread use of torture.

In his evening video address on Saturday, Zelensky said at least 10 torture sites had now been discovered in the liberated eastern region.

"Torture was a widespread practice in the occupied territory," the president said, calling the occupiers, who fled in the face of a Ukrainian counteroffensive a week ago, "fascists" and comparing their behavior to that of the Nazis.

Like the Nazis, the Russians would be held accountable for their actions in court, Zelensky said. "We will establish the identities of all those who tortured and abused people, who brought these atrocities from Russia here to Ukrainian territory."

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He said the occupying forces had abandoned torture equipment when they fled. Ukrainian authorities have also published photos of what they believe to be torture sites and equipment.

Evidence of torture had also been found among the bodies discovered in a mass grave in a forest near the city of Izyum, Zelensky said, adding that work exhuming the dead had continued on Saturday.