LONDON: British Prime Minister Boris Johnson on Wednesday called Russian President Vladimir Putin a war criminal and urged UN unanimity to condemn his invasion of Ukraine.

His comments came in parliament, where MPs gave a standing ovation to Ukraine's ambassador in attendance.

Johnson wore a UK-Ukraine flag pin, and many lawmakers wore clothing in Ukraine's blue and yellow colours.

"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," Johnson said.

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UK ministers have been warning that Putin, his cronies and commanders could face prosecution at the International Criminal Court in The Hague, which has already opened a probe into the invasion.

Ahead of a vote by the UN General Assembly in New York, Johnson added: "We call on every nation to join us in condemning Russia and demanding that Putin turns his tanks around."

Johnson warned of further sanctions if the offensive continues, reprising a new three-word slogan: "Putin must fail".

"With the heroism of Ukraine's people... and the unity of the West, I've no doubt that he will fail and we will succeed in protecting Ukraine," Johnson said.