The UK has condemned Russia's "reprehensible" nuclear threats and said the mobilisation of reserves was a sign its invasion of Ukraine is "failing".
In an extraordinary televised address, Vladimir Putin claimed that the West was engaged in nuclear blackmail against Russia and that he is unafraid to retaliate with "lots of weapons."
The Russian President, who continually misrepresents the illegal invasion of Ukraine, warned his threats were "not a bluff".
He ordered a "partial mobilisation" of Russia's military reserves, which defence minister Sergey Shoigu said amounted to around 300,000 troops.
Downing Street condemned the Kremlin's "reprehensible actions" and said Putin's desperate call-up to reservists showed the invasion was stuttering.
A spokeswoman for the Prime Minister said: "Ukraine is one of the key focuses of the UN GA meeting. Putin's speech this morning and his move to mobilise the Russian population are a clear admission that his invasion is failing.
"The UK alongside our international partners stand united in condemning the Russian Government's reprehensible actions.
"We will continue to support the Ukrainian Government in the face of this assault on their sovereignty."
Defence Secretary Ben Wallace said Mr Putin's actions were "an admission that his invasion is failing".
"No amount of threats and propaganda can hide the fact that Ukraine is winning this war, the international community are united and Russia is becoming a global pariah."
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.
Liz Truss is expected to speak out in support of Ukraine later today when she addresses the UN General Assembly in New York.
The Prime Minister will give a speech which is expected to focus on foreign policy and defence spending on her first major diplomatic outing.
