US President Joe Biden said the world is facing nuclear “Armageddon”, warning that Vladimir Putin may use his atomic arsenal as Russian troops struggle against a Ukrainian counter-offensive.

After Russia’s invasion eight months ago, Putin has made thinly veiled threats to use nuclear weapons if he feels he has run out of options in his bid to seize swaths of Ukrainian territory in the face of stiff resistance by Western-back Kyiv.

While experts say any nuclear attacks would likely be relatively small, Biden warned that even a tactical strike in a limited area would still risk triggering a wider conflagration.

Ukraine’s proclaimed wins in the southern region of Kherson are the latest in a series of Russian defeats undermining the Kremlin’s claim to have annexed around 20 percent of Ukraine.

The recaptured territory was home to dozens of towns and villages that had been occupied by Russian forces for months.