On a visit to Ukraine days after a meeting with Russia’s foreign minister in Moscow, Mr Guterres was visibly moved by the sight of shelled apartment blocks in Borodyanka.
“When I see those destroyed buildings, I imagine my family in one of those houses now destroyed and black. I see my granddaughters running in panic," he said.
“The war is an absurdity in the 21st century – the war is evil and when you see these situations our heart of course stays with the victims, our condolences to their families.
“But our emotions - there is no way a war can be acceptable in the 21st century.”
Mr Guterres has also visited Bucha, where scenes of civilians’ bodies laying in the streets after Russia’s withdrawal from the town shocked the world and led to Russia being suspended from the UN’s top human rights body.
He is expected to meet Ukraine’s president Volodymyr Zelensky later Thursday in Kyiv.
The trip comes days after a visit to Moscow for a meeting and press conference with Russia’s Sergei Lavrov.
Speaking at a press conference Tuesday, Mr Guterres vowed the UN was ready to mobilise all of its resources to help save civilian lives in the besieged southern port city of Mariupol.
He also called for the establishment of safe humanitarian corridors to allow civilians to escape fighting and go in any direction they choose.
His visit comes after the UK’s Defence Secretary Ben Wallace warned that Putin may “fortify and dig in” in Ukraine.
He told Sky News: “It’s certainly the case that Putin, having failed in nearly all of his objectives, may seek to consolidate what he’s got … And just be a sort of cancerous growth within the country of Ukraine and make it very hard for people to move them out of those fortified positions.
“If we want this to not happen, we have to help Ukraine try to get the limpet off the rock and keep the momentum pushing them back.”