Vladimir Putin is desperately trying to justify a "catastrophic" failure in Ukraine, Liz Truss has said, accusing the Russian president of "sabre rattling" after he warned his country would use "all the means at our disposal" to protect itself.
It comes as world leaders gather at a United Nations summit in New York to discuss the ongoing Russian assault on Ukraine.
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.
Ms Truss, addressing the United Nations General Assembly, praised the "strength of collective purpose" in response to Mr Putin's invasion so far, but warned that support for Ukraine must not wane.
"In Ukraine, barbarous weapons are being used to kill and maim people. Rape is being used as an instrument of war. Families are being torn apart," she said.
"And this morning we have seen Putin desperately trying to justify his catastrophic failures.
"He is doubling down by sending even more reservists to a terrible fate.
"He is desperately trying to claim the mantle of democracy for a regime without human rights or freedoms.
"And he is making yet more bogus claims and sabre-rattling threats.
"This will not work. The international alliance is strong – Ukraine is strong."
British diplomats were taking Mr Putin’s nuclear threats seriously, but the thinking was they have already seen him lie and bluff during the war.
Meanwhile, in an update on Twitter, the Ministry of Defence said Putin's plans to mobilise an additional 300,000 reservist troops would be a "struggle".
Watch: What will the next 90 days of the war in Ukraine look like?
"Russia is likely to struggle with the logistical and administrative challenges of even mustering the 300,000 personnel," the update said.
"It will probably attempt to stand up new formations with many of these troops, which are unlikely to be combat effective for months."
The MOD said "even this limited mobilisation is likely to be highly unpopular with parts of the Russian population".
"Putin is accepting considerable political risk in the hope of generating much needed combat power.
"The move is effectively an admission that Russia has exhausted its supply of willing volunteers to fight in Ukraine."
Defence Secretary Ben Wallace also said Mr Putin's actions were "an admission that his invasion is failing" and "Russia is becoming a global pariah".