Allowing Russian President Vladimir Putin to succeed in invading Ukraine would have "absolutely disastrous" consequences for the world, British Prime Minister Boris Johnson warned in an interview with CNN on Sunday.

Speaking hours after Russian missiles struck Kiev, Johnson urged Americans, Britons and other Western nations to remain determined to punish Moscow despite the war's impact on global oil prices.

"I would just tell people in the United States that this is what America has historically done and should do, which is to stand up for peace and freedom and democracy," Johnson said.

"And if we let Putin get away with it and (let) just annex, conquer significant parts of a free, independent, sovereign country, which he is going to do, then the consequences for the world will be absolutely disastrous," the British prime minister added.