Russia will use its largest and most-powerful nukes to defend the territory it occupies in Ukraine, a staunch ally of Vladimir Putin has insisted today.

Dmitry Medvedev, who served as a stand-in for Putin from 2008 to 2012, vowed the territories will become part of Russia when referendums are held this week – after which ‘any Russian weapons’ will be used to defend them. 

That includes ‘strategic nuclear weapons‘ such as Putin’s giant new Sarmat missile, Medvedev said, and ‘weapons based on new principles’ – likely a reference to hypersonic technology that the Kremlin claims is invulnerable to air defenses. ‘The Western establishment [and] all citizens of the NATO countries need to understand that Russia has chosen its own path. There is no way back,’ he added.

Medvedev spoke out a day after Putin gave a fire-and-brimstone speech in which he issued a fresh nuclear threat against the West, telling world leaders: ‘I’m not bluffing.’

But Liz Truss, the UK Prime Minister, hit back in a speech at the UN last night – telling Putin the West would not be ‘cowed’ by nuclear blackmail.

She vowed billions more in support for Ukraine, and said that Britain would not stop arming the country until Russia is defeated.

US President Joe Biden was similarly defiant, pledging the West will ‘stand in solidarity against Russia’s aggression, period.’

He added: ‘A nuclear war cannot be won and must never be fought.’

Putin also announced the ‘partial mobilisation’ of Russia’s military reserves yesterday, including those with combat experience and ‘special skills’ the army needs.

It marks the first time since World War Two that Russia has press-ganged its population into the army, and comes in the wake of another humiliating defeat.

Ukraine last week scored a major victory by routing Russia’s army to the east of Kharkiv, leaving troops fighting for control of the Donbas vulnerable.

Sergei Shoigu, the Russian defence minister, said 300,000 Russian men will now be conscripted and sent to the front – twice the size of the initial invading force – in the hopes of stopping the rot and capturing the whole of the Donbas region.