Liz Truss has blasted Vladimir Putin for “desperately trying to justify his catastrophic failures” as he “doubled down” on the Ukraine war by calling-up reservists.
The Prime Minister addressed the United Nations General Assembly in the early hours of the morning, telling world leaders of the horrors unfolding in Ukraine.
Speaking 24 hours after meeting Ukrainian First Lady Olena Zelenska, Ms Truss told the UN: “As we meet here this evening, 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.
“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.
“The contrast between Russia ’s conduct and Ukraine’s brave, dignified First Lady, Olena Zelenska, who is here at the UN today could not be more stark.
“The Ukrainians are not just defending their own country – they are defending our values and the security of the whole world.
“That’s why we must act.”
Ms Truss hailed the “resolute international response” to Russia’s invasion which “has shown how we can deliver decisive collective action”.
She added: “The response has been built on partnerships and alliances, but also on being prepared to use new instruments – unprecedented sanctions, diplomatic action, and rapid military support.”
Foreign Secretary James Cleverly met his Russian counterpart Sergei Lavrov at the United Nations on Thursday.
Mr Cleverly highlighted Ukrainians’ brave resistance to Russian invaders who waged war on their neighbour on February 24.
He told the council: “Ukrainians’ resilience and spirit of defiance, in defence of their country, continues to inspire all free people and nations.
“But every day, the devastating consequences of Russia’s invasion become more clear and evidence of Russian atrocities continues to mount.
“We can and must make clear to President Putin that his attacks on the sovereign will of the Ukrainian people - so clearly expressed as they fight for their homes - must stop.
“His assaults on the UN charter and international norms that protect us will not be tolerated and that he must withdraw from Ukraine to enable a return to regional and global stability.”
Mr Cleverly was also expected to accuse Moscow of planning to fix the results of “sham referendums” in annexed regions that Vladimir Putin is set to to exploit to claim his invasion is justified.
The Foreign Office said Mr Cleverly will “reveal evidence of Russian tactics used to subvert Ukrainian democracy and rob Ukrainians of their homes and identity”.
Before addressing the UN, Ms Truss held talks with European Commission President Ursula von der Leyen.
The pair branded Putin’s call-up of reservists a “statement of weakness”.
A No10 spokeswoman said: “They strongly condemned Russia’s actions in Ukraine and agreed that Putin’s recent calls to mobilise parts of the population were a sign that Russia's invasion is failing. It is a statement of weakness.”
Earlier, US President Joe Biden accused Russia of violating core principles of UN membership by invading Ukraine.
He warned that Moscow was making "irresponsible" threats to use nuclear weapons.
“A permanent member of the United Nations Security Council invaded its neighbour, attempted to erase a sovereign state from the map. Russia has shamelessly violated the core tenets of the United Nations Charter,'' Mr Biden told the UN General Assembly.
He added: "A nuclear war cannot be won and must never be fought.”
Earlier Ms Truss and Mrs Zelenska toured a moving exhibition detailing Russian war crimes.
The PM and First Lady visited the Ukrainian Institute of America in New York, where they are attending the United Nations General Assembly.
The 12 day exhibition, which runs until October 2, begins with an “incomplete map of Russian war crimes in Ukraine by September 11”.
It pinpoints locations where, it says, Russia has committed atrocities since invading on February 24 - killing 7,195 civilians, including 383 children.
Ms Truss said: "These are the types of crimes we thought had been consigned to history.”
Curators say no data exists for Mariupol as it cannot be verified because of the situation on the ground in the war-ravaged southern port city.
The exhibition includes a photo taken in Kharkiv in July showing a woman in a yellow dress outside a bombed out block of flats.
She went out to feed her cat but was caught in Kremlin shelling and killed.
A shoe remains only on her left foot.
Her husband, wearing a blue t-shirt and red baseball cap, sits on a bench weeping while green leaves lie on the ground underneath a tree.
Curators studied 6,683 images before selecting 22 to exhibit at the display, which has also been shown at NATO headquarters and the European Parliament in Brussels.
They include a picture of a pregnant woman cradling her bump being carried on a stretcher covered in strawberry blanket.
Another shows dead bodies wrapped in sheets being piled in a trench.
As the brief tour concludes, an eight-minute, 50-second film of harrowing war images is played on loop.
Over the gruesome footage is played a haunting soundtrack of intercepted calls between Russians soldiers and their fellow invading troops, or bugged clips of them boasting to their families of rape, looting and torture inflicted on their neighbours.
