Former Deputy Supreme Allied Commander Europe Richard Shirreff told BBC Radio's "Today" that the West must prepare for a "worst-case" scenario in the ongoing Russian invasion of Ukraine — war with the Kremlin, Yahoo News reported.

"The worst case is war with Russia," Shirreff told the radio network. "By gearing itself up for the worst case, it is most likely to deter [Russian President Vladimir] Putin because ultimately Putin respects strength."

Shirreff also endorsed comments made by U.K. Foreign Secretary Liz Truss that NATO needs to "double down" on its support for Ukraine.

"It's got to be followed through with significant resources, and it's got to be done right across the alliance as a whole," he said.

The comments by Shirreff follow U.N. Secretary-General António Guterres' visit to areas around Kyiv that were formerly held by Russian forces on Thursday.

"When I see those destroyed buildings ... I imagine my family in one of those houses that is now destroyed and black. I see my granddaughters running away in panic," Guterres said while visiting Borodyanka.

"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," he continued. "But our emotions are ... there is no way a war can be acceptable in the 21st century."

Guterres visited Bucha later in the morning, the site of what Ukraine claimed is the massacre of Bucha civilians by Russian forces. He told reporters that he fully supports the International Criminal Court's investigation of the alleged war crimes.

"When we see this horrendous site, it makes me feel how important it is [to have] a thorough investigation and accountability," the secretary-general said. "But when we talk of war crimes, we cannot forget that the worst of crimes is war itself."

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