Evidence suggests Russia is planning "the biggest war in Europe since 1945", Prime Minister Boris Johnson has said, APA reports citing BBC.
He told the BBC's Sophie Raworth in an interview: "All the signs are that the plan has already in some senses begun."
Intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv, Mr Johnson said.
"People need to understand the sheer cost in human life that could entail," he said.
The prime minister was speaking from Munich, where world leaders are meeting for an annual security conference.
The latest estimates by the US government suggests that between 169,000 and 190,000 Russian troops are now stationed along Ukraine's border, both in Russia and neighbouring Belarus - but this figure also includes rebels in eastern Ukraine.
Mr Johnson also indicated that the UK would bring in even more far-reaching sanctions against Russia than have been suggested before.
He said the UK and the US would stop Russian companies "trading in pounds and dollars" - a move that he said would "hit very very hard" with its impact.
Previous sanctions suggested by ministers include broadening the range of Russian people and businesses the UK could target.