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British Foreign Secretary Liz Truss said she was ‘horrified’ on Sunday after a Russian bomb hit a Ukrainian school where 90 people were sheltering.
Around 60 people are feared dead after the school in Bilohorivka in the Luhansk region of eastern Ukraine was bombed and caught fire on Saturday. Only 30 people were saved.
The foreign secretary said that Vladimir Putin’s regime in Moscow would be held to account for the ‘war crime’.
“Horrified by Russia’s latest attack on a school in Luhansk, resulting in the deaths of innocent people sheltering from Russian bombardment," Ms Truss tweeted.
She said the deliberate targeting of civilians and infrastructure “amounts to war crimes” and “we will ensure Mr Putin’s regime is held accountable”.
Mr Johnson told his counterparts “the world must go further and faster to support Ukraine” against the Kremlin’s invading forces.
“The prime minister said there was a savage irony that the leaders had gathered to discuss Putin’s barbaric invasion on a day when they should be remembering the sacrifice of Russian soldiers defeating fascism in the Second World War," a Downing Street spokeswoman said.
“He agreed with G7 leaders that the world must intensify economic pressure on Putin in any way possible, and said the West must not allow the war to turn into a stalemate that only magnified suffering.
“Ukraine needed to receive military equipment that allowed them to not only hold ground in Ukraine, but also recapture it, the prime minister told the leaders.”
The G7 agreed more needed to be done to support Ukraine’s agricultural exports, with the war affecting food supplies around the world.
Mr Johnson also urged G7 countries to intensify diplomatic lobbying of countries which were failing to apply pressure on Moscow, “especially as it was clear grossly unjustifiable human rights abuses and war crimes were being committed”, the spokeswoman added.
The UK’s Ministry of Defence suggested Russian forces were struggling because of the high casualty rate among Moscow’s commanders.
“Difficulties in command and control, as well as faltering Russian performance on the front line, have drawn senior commanders on to the battlefield, likely to take personal leadership of operations,” a defence intelligence update said.
This has put them at risk of being attacked, with “disproportionately high losses of Russian officers in this conflict”.
“This has resulted in a force that is slow to respond to setbacks and unable to alter its approach on the battlefield,” the update said.
“These issues are likely to endure given the relative lack of operational command experience of the officers promoted in place of those killed.”
US officials claimed two major Russian tank factories had ground to a halt because of the impact of economic sanctions.
The UK has pledged an extra £1.3 billion ($1.6bn) in military support to Ukraine, in a dramatic escalation of assistance for Mr Zelenskyy’s forces as they fight the Russian invasion.
It is the highest rate of UK military spending on a conflict since the height of the campaigns in Iraq and Afghanistan.
The £1.3bn, drawn from the UK’s reserves, includes £300 million ($370m) of military kit promised by Mr Johnson earlier this week. The kit includes anti-battery radar systems to attack Russian artillery, GPS jamming equipment and night-vision devices.
The prime minister will host a meeting of arms companies later this month to discuss increasing production in response to the demand created by the conflict in Ukraine.
Officials said the announcement would help to support the British arms industry, which could benefit from the global shift away from reliance on equipment from sanctions-hit Russia.
“Putin’s brutal attack is not only causing untold devastation in Ukraine, but it is also threatening peace and security across Europe," said the prime minister.
“The UK was the first country to recognise the scale of the threat and send arms to help the Ukrainians defend themselves.
“We will stand by that endeavour, working with our allies to ensure Ukraine can continue to push back the Russian invasion and survive as a free and democratic country.
“In the process, we are bolstering our own security and economy, turbocharging the development and production of cutting-edge defence equipment here in the UK.”