The "biggest war in Europe since 1945" is being planned by Russia, UK Prime Minister Boris Johnson says, as the US warns Ukraine could be attacked at any moment.
Johnson's comments come after Russia's strategic nuclear forces held exercises overseen by President Vladimir Putin on Saturday.
"The plan that we're seeing is for something that could be, really, the biggest war in Europe since 1945 - in just in terms of sheer scale," Johnson told the BBC in an interview.
"I think people need to understand the sheer cost in human life that could entail - not just for Ukrainians but also for Russians."
Ukraine has reported an increase in ceasefire violations by Russia-backed separatists recently, and Kyiv's Western allies are concerned Russia is poised to launch a military offensive against Ukraine. Russia denies any plans to attack its neighbour.
Foreign ministers from the G7 group of rich nations including Canada, France, Germany, Italy, the UK and the US said it sees no evidence Russia is reducing military activity near Ukraine's borders and remains "gravely concerned" about the situation.
The G7, in a statement released by the UK's foreign ministry, said it was calling on Russia to "choose the path of diplomacy, to de-escalate tensions, to substantively withdraw military forces from the proximity of Ukraine's borders and to fully abide by international commitments".
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"As a first step, we expect Russia to implement the announced reduction of its military activities along Ukraine's borders. We have seen no evidence of this reduction," the G7 added.
On Sunday, the White House said Russia could launch an attack against Ukraine at any time - adding US President Joe Biden is due to convene a National Security Council meeting to discuss the situation.
Biden said said on Friday he believed Putin would invade Ukraine in the coming days.
"As of this moment, I am convinced that he has made the decision," Biden told reporters at the White House.
Ukraine President Volodymyr Zelenskiy said on Saturday he had an "urgent" phone conversation with France's Emmanuel Macron and discussed possible ways of immediate de-escalation and political-diplomatic settlement in eastern Ukraine.
Two regions in eastern Ukraine where government and separatist forces have been fighting since 2014 were hit by more than 1400 explosions on Friday.
