Senior Russian security official Dmitry Medvedev said on Tuesday that if the West supplied nuclear weapons to Ukraine then Moscow could consider such a transfer to be tantamount to an attack on Russia,

Putin lowered the bar for a nuclear attack after the US lifted a ban on Ukraine using long-range missiles. Ukraine carried out its first strike with the weapons.

The new doctrine allows for a potential nuclear response by Moscow even to a conventional attack on Russia by any nation that is supported by a nuclear power.

The war between Russia and Ukraine is intensifying with new attacks and new concerns over the weapons being used by both sides. Vladimir Putin says Russia’s use of a nuclear-capable weapon is a response to Ukraine firing American weapons deeper into Russia for the first time.

Deploying British troops to Ukraine could be like a "declaration of war against Russia" and cause a "nuclear exchange", an expert has told the Daily Express UK.