Evidence suggests Russia is planning "the biggest war in Europe since 1945", UK Prime Minister Boris Johnson has said.

“All the signs are that the plan has already in some senses begun,” he told the BBC in an interview on February 20.  

According to him, intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv.

"People need to understand the sheer cost in human life that could entail," Mr. Johnson said.

The prime minister was reportedly speaking from Munich, where world leaders gathered for an annual security conference.

The latest estimates by the US government suggests that between 169,000 and 190,000 Russian troops are stationed along Ukraine's border, both in Russia and neighboring Belarus - but this figure also includes rebels in eastern Ukraine.

Mr. Johnson warned that any conflict could be "bloody and protracted", saying Russian President Vladimir Putin was possibly “thinking illogically about this" and did not "see the disaster ahead.” 

“I think it's vital for us all now to get over what a catastrophe it would be for Russia,” he added.

He indicated that the UK and US would bring further sanctions against Russia than have been suggested before, including stopping its companies "trading in pounds and dollars" - a move that he said would "hit very, very hard".

Mr. Johnson added: "The lesson of [the Russian seizure of Crimea from Ukraine in] 2014 is that you can't just let Vladimir Putin get away with it.”

An invasion of Ukraine would strengthen, rather than weaken, NATO, he predicted, saying Western countries could not allow opposition forces to come to "the false conclusion that might is right".