UK Prime Minister Boris Johnson on Thursday warned Russia in stark terms against any military action in Ukraine, as Britain joined the US and European allies for crisis talks in Berlin.

Johnson spoke after US President Joe Biden sparked controversy by appearing to indicate that a "minor incursion" by Russia might prompt a smaller response from NATO allies. The White House downplayed the remark.

The prime minister said: "Be in no doubt that if Russia were to make any kind of incursion into Ukraine, on any scale whatever, I think that would be a disaster not just for Ukraine but for Russia.

"It would be a disaster for the world," he told reporters.

Britain stands "four-square" behind Ukraine's sovereignty, Johnson added, as US Secretary of State Antony Blinken opened the talks in the German capital seeking a common strategy against Moscow.