LONDON: The West will need an overwhelming display of unity if it is to persuade Russian President Vladimir Putin to avoid a "catastrophic" invasion of Ukraine, British Prime Minister Boris Johnson said on Saturday.

He made the comments before visiting the Munich Security Conference, which has been dominated by the crisis over Ukraine and Western concern that Russia is poised to invade its neighbour.

"There is still a chance to avoid unnecessary bloodshed, but it will require an overwhelming display of western solidarity beyond anything we have seen in recent history," Johnson said in a written statement to media.

The three-day Munich meeting, which began on Friday, has been attended by dignitaries including US Vice President Kamala Harris and NATO Secretary-General Jens Stoltenberg.

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"I'll be urging unity in the face of potential Russian aggression in Ukraine. And that unity is absolutely vital if we're going to deter what I think would be an absolutely catastrophic act of aggression by Vladimir Putin," Johnson said in a video on social media.

Johnson's office said he would deliver a similar message in his speech at the conference, and while in Munich would also meet with several European partners to discuss the response to the Ukraine crisis.

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Russian-backed separatist leaders in eastern Ukraine declared a full military mobilisation on Saturday, a day after ordering women and children to evacuate to Russia, citing the threat of an imminent attack by Ukrainian forces.

Kyiv flatly denied the accusation and Washington said it was part of Russia's plan to create a pretext for an invasion of Ukraine. Moscow denies having plans to invade.