Vladimir Putin’s warning that Russia will use “all the means at our disposal” to protect itself has been dismissed as “sabre rattling” by Prime Minister Liz Truss.

The Russian president’s comments in a televised address to the nation appeared to suggest the conflict in Ukraine could spiral into a nuclear crisis.

He announced a partial military mobilisation, with 300,000 reservists set to be called up as the Kremlin attempts to regain ground in the face of a counter-attack by Ukraine’s forces.

And Mr Putin said “it’s not a bluff” when he vowed that Russia would use its weapons of mass destruction if its territory was threatened.

The Russian leader accused the West of “nuclear blackmail” and claimed “high-ranking representatives of the leading Nato states” had talked about the possibility of using weapons of mass destruction against Russia.

“To those who allow themselves such statements regarding Russia, I want to remind you that our country also has various means of destruction,” Mr Putin said.

Ms Truss, speaking in New York where she is attending the United Nations General Assembly, said it was important for her to be at the gathering with fellow leaders “because we are facing such a difficult international situation with the war in Ukraine started by Vladimir Putin”.

“We’ve heard more sabre rattling from him this morning,” she said.

“But also we need to get the global economy back on its feet after Covid, and really ensure democracy prevails.”

In a joint statement with European Commission president Ursula von der Leyen she added that the Russian military mobilisation was “a statement of weakness”.

Moscow-controlled regions in eastern and southern Ukraine are set to hold referendums on becoming parts of Russia, which could give the Kremlin the pretext for a wider war because Mr Putin would be able to claim parts of his state were being attacked.

Foreign Secretary James Cleverly said the UK would never recognise the results of “sham referendums” which were “held at the barrel of a gun”.

Defence Secretary Ben Wallace said Mr Putin’s actions were “an admission that his invasion is failing” and “Russia is becoming a global pariah”.

Melinda Simmons, the UK’s ambassador in Kyiv, said the Russian president’s “essential weakness” was “he still refuses to understand Ukraine”.

A British defence intelligence update suggested Mr Putin was being forced to undermine his own public position that the war in Ukraine was a “special military operation” rather than a full-scale conflict.

“These new measures have highly likely been brought forwards due to public criticism and mark a further development in Russia’s strategy,” the Ministry of Defence said.

“Putin is accepting greater political risk by undermining the fiction that Russia is neither in a war nor a national crisis in the hope of generating more combat power.”

A former adviser to Mr Putin suggested the Russian leader would be ready to use nuclear weapons against western nations such as the UK.

On whether nuclear missiles could be targeted at London, Russian political scientist Sergei Markov claimed on BBC Radio 4’s Today programme “yes, it is possible”.

Mr Markov added: “This nuclear war could be a result of the crazy behaviour of the president of the United States Joe Biden and prime ministers of Great Britain Boris Johnson and Liz Truss.”

But Evgeny Popov, a member of the Duma for Mr Putin’s United Russia party, told the BBC: “We are not going to attack western countries first. We are not going to do some nuclear massacre in the world.

“It’s not our policy. It’s not our practice. We are peaceful people in Russia, but we can respond if you would attack us.”

The Prime Minister, who will hold talks with the US president on Wednesday, was using her visit to the United Nations to rally support for Ukraine.

Ms Truss met Ukrainian first lady Olena Zelenska and prime minister Denys Shmyal to tour an exhibition titled “Russian Warcrimes” at the Ukrainian Institute of America on Tuesday evening.