Boris Johnson's comments came at a joint press conference with Polish Prime Minister Mateusz Morawiecki
Vladimir Putin is engaging in "barbaric and indiscriminate" tactics to target civilians in Ukraine, Prime Minister Boris Johnson has said.
He also said the Russian president had "fatally underestimated" the willingness of the Ukrainian people to fight and the resolve of the West.
The UK is considering calling for Russia to be expelled from the UN Security Council, the BBC understands.
The prime minister was speaking in Poland as he meets key European allies.
His comments came in a press conference with his Polish counterpart Mateusz Morawiecki, where he said he was "increasingly confident" Russia's invasion would fail, adding that Mr Putin "must fail" in his aims in Ukraine.
Mr Johnson said Mr Putin was prepared to "bomb tower blocks, to send missiles into tower blocks, to kill children, as we are seeing in increasing numbers".
He also paid tribute to the "leadership and courage" of Ukrainian President Volodymyr Zelensky, adding: "I think he has inspired and mobilised not only his own people, he is inspiring and mobilising the world in outrage at what is happening in Ukraine."
Mr Johnson is set to warn in a speech that Ukraine's occupation would have "grave" consequences for the global order.
The Russian army continues its advance on Kyiv, with satellite images spotting an armoured convoy about 40 miles long.
Air-raid sirens were sounded again in the capital overnight, and there were also reports of fierce shelling in other cities including Chernihiv in the north.
The fighting escalated on Monday, with missiles killing dozens of civilians in Kharkiv, Ukraine's second city. Mr Zelensky has described the bombardment as a war crime.
In other developments:
Mr Johnson will later meet Estonian leaders and Nato secretary-general Jens Stoltenberg and is due to discuss the response to the unfolding humanitarian crisis on Ukraine's border, as well as European security.
He will also speak to British troops serving in Estonia - which shares a border with Russia.
The prime minister is set to promise further sanctions on Mr Putin's allies and tell the British people that there will be a cost to the economy, for example in higher energy bills, the BBC understands.
He will also say there are 1,000 British troops on standby to help with the humanitarian response in Europe.
Meanwhile, Deputy Prime Minister Dominic Raab has warned that Russian President Vladimir Putin could retaliate with increasingly "heavy-handed tactics" if his invasion of Ukraine faces further hitches.
He told BBC Radio 4's Today programme: "We can expect, for every stutter and stumble, him to try and come back for even more heavy-handed tactics, but that is a sign that the initial phase at least - and this is going to be a long haul - has not lived up to his expectations."
Shadow defence secretary John Healey said the Labour party would support the UK government in responding to the invasion, adding: "The most important thing is there is a united UK voice, standing with the Ukrainians."
Military tactics researcher Dr Jack Watling said the West was trying to provide aide far too late and as a result that aide, including arms shipments announced over the last few days, was "essentially building up in Poland".