Vladimir Putin’s nuclear threats are part of a desperate attempt to justify his “catastrophic” failure in Ukraine, Liz Truss is to say.

In her first speech on the world stage as prime minister, Ms Truss will accuse the Russian president of “sabre rattling” after he warned his country would use “all the means at our disposal” to protect itself.

The comments appeared to suggest the conflict in Ukraine could spiral into a nuclear crisis.

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Ms Truss will say Mr Putin is “desperately trying to justify his catastrophic failures” as she delivered an address to the United Nations General Assembly (Unga) in New York.

“He is doubling down by sending even more reservists to a terrible fate,” she will say.

“He is desperately trying to claim the mantle of democracy for a regime without human rights or freedoms.

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“And he is making yet more bogus claims and sabre-rattling threats.”

Ms Truss will praise the “strength of collective purpose” in response to Mr Putin’s invasion so far, but warned that aid for Ukraine must not wane.

And she will tell fellow world leaders that the UK will spend 3% of GDP on defence by 2030, repeating a promise she made when she campaigned to become Tory leader.

She will add: “In the face of rising aggression we have shown we have the power to act and the resolve to see it through. But this must not be a one-off.

“This must be a new era in which we commit to ourselves, our citizens, and this institution that we will do whatever it takes – whatever it takes to deliver for our people and defend our values.”

The speech comes against the backdrop of protests in Moscow after Mr Putin announced a partial military mobilisation, with 300,000 reservists set to be called up.

It brought people onto the streets with more than 1,000 arrests made as of Wednesday evening, according to a rights group.

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The Kremlin is attempting to regain ground in the face of a counter-attack by Ukraine’s forces.

In a televised address to the nation, 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.

Nuclear threats ‘taken very seriously’

The comments prompted stern responses from other world leaders, with Joe Biden using his speech at the UN General Assembly to accuse Russia of violating the core tenets of the United Nations charter with its “brutal, needless war” in Ukraine.

A diplomatic source said the nuclear threat is “very real” and being taken “very seriously” by officials.

But they added: “We’re not going to engage in a battle of words about a nuclear threat from a man who postponed his speech last night, was shaking when he delivered it, and is now attempting to mobilise reservists who are attempting to get to airports across Russia. His lies are catching up with him.”

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Ms Truss has been using her visit to the United Nations to rally support for Ukraine.

However, she has not managed to escape the thorny issue of the Northern Ireland protocol.

She discussed the issue in meetings with Mr Biden and Ursula von der Leyen on Wednesday.

It is understood the prime minister wants to break the impasse over the protocol within the next six months, amid escalating tensions apparent on her trip.

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While there was no major breakthrough on Northern Ireland following the meetings, the prospect of a presidential state visit to the UK to coincide with the 25th anniversary of the Good Friday Agreement next Easter has been touted.

One senior diplomatic source said: “If you look at the calendar you will conclude that next year might be an obvious reason to visit Europe if things come good.”

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