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Britain and dozens of allies are pressing for an investigation to be launched rapidly into Vladimir Putin’s bombardment of Ukrainian cities after Prime Minister Boris Johnson accused the Russian president of war crimes.
Mr Johnson said Mr Putin “cannot commit these horrific acts with impunity” after 37 countries joined the UK on Wednesday in referring Moscow to the International Criminal Court.
The move, allowing the ICC’s prosecutor to proceed straight to investigation without the need for judicial approval, came as Ukraine’s capital Kyiv braced for a siege.
The country's second-largest city Kharkiv reeled from further strikes and the control of port city Kherson was contested.
Strikes that damaged the Babyn Yar Holocaust memorial in Kyiv and the central square in Kharkiv have caused revulsion.
Western allies fear it indicates a shift in Russian tactics towards indiscriminate strikes on urban areas.
For the first time, the UK explicitly accused Mr Putin of war crimes, with Downing Street claiming “horrific acts” were occurring on an almost hourly basis as population centres were made targets.
“Putin has gravely miscalculated in his abhorrent assault on a sovereign nation," Mr Johnson said at Prime Minister’s Questions.
"He has underestimated the extraordinary fortitude of the Ukrainian people and the unity and resolve of the free world in standing up to his barbarism.
“What we have seen already from Vladimir Putin’s regime in the use of the munitions that they have already been dropping on innocent civilians, in my view, already fully qualifies as a war crime."
More than 2,000 civilians have died since the invasion, Ukraine’s state emergency service said, although that figure has not been independently verified.
The UN refugee agency believes 874,000 people have fled Ukraine but that figure is soon expected to reach a million.
ICC chief prosecutor Karim Khan said on Monday that he planned to open an investigation into events in Ukraine “as rapidly as possible”.
But the UK Foreign Office said the largest referral of its kind from the 38 allies, including Ireland, Canada, France, Germany, Italy, Spain and New Zealand, would allow him to “proceed straight to an investigation, without the need for judicial approval”.
“Putin’s military machine is targeting civilians indiscriminately and tearing through towns across Ukraine," Foreign Secretary Liz Truss said.
“An investigation by the International Criminal Court into Russia’s barbaric acts is urgently needed and it is right that those responsible are held to account.
“The UK will work closely with allies to ensure justice is done.”
Justice Secretary Dominic Raab, a former lawyer who has prosecuted war crimes, said it was now critical to “preserve properly all evidence of war crimes”.
“Any Russian leader or officer carrying out orders that amount to war crimes should know they face ending up in the dock of a court and ultimately in prison," Mr Raab said.
Earlier, Mr Johnson promised to publish a list of people associated with Mr Putin on whom sanctions could be imposed, as “the vice is tightening on the Putin regime” through the restrictions introduced by the West.
But he was urged by Labour leader Sir Keir Starmer to immediately increase the measures against allies of the Russian leader.
“Now is the time to sanction every oligarch and crack open every shell company so we can prove Putin wrong,” said Mr Starmer, who called for Chelsea FC owner Roman Abramovich to face sanctions.
The Russian-Israeli billionaire later said he would sell the club, with the “net proceeds” going to a charity supporting victims of the war in Ukraine.
UK officials have said more sanctions are coming against oligarchs, Russian National Security Council members and banks, and that they believe the economic shock has been more significant than Mr Putin was expecting.
They said they wanted sanctions to go further, including banning all Russian banks from the Swift payment system.