Vladimir Putin has threatened a "nuclear response" to any "joint attack" on Russia by Ukraine supported by a Western ally.

In a dire warning issued on Tuesday, Moscow said it "reserved the right" to use nuclear weapons in the event of "aggression" against Russia or Belarus with conventional weapons if it creates a "critical threat to their sovereignty".

The threat came as Putin signed Russia's new nuclear doctrine into force - which the Kremlin said was necessary to make potential enemies understand the inevitability of retaliation for an attack on Russia or its allies.

Kremlin spokesman - and Putin's spin doctor - Dmitry Peskov called the new doctrine a "very important text", with the Russian premier signing off on it just one day after US counterpart Joe Biden approved the use of American long-range weapons by Ukraine.

The threat came as Vladimir Putin signed Russia's new nuclear doctrine into force

Starmer has been lobbying the US to let Britain equip Ukraine with Storm Shadow missiles for months

On Monday, Russia called Biden's sign-off "reckless and dangerous" - with Peskov taking a pot-shot at the outgoing Biden for "adding fuel to the fire and continue to provoke tension around this conflict".

Security sources said the Americans' green-light for Ukraine to use US-made ATACMS missiles came in response to the deployment of North Korean troops in Russia to help fight off Ukraine's incursion into Kursk.

But Biden has not yet signed off on Britain's own push to equip Ukraine with similarly long-range Storm Shadow missiles - despite months of lobbying by Sir Keir Starmer's Government.