British Prime Minister Boris Johnson and U.S. President Joe Biden have concluded that diplomacy and Russia have a crucial window to retreat from threats to Ukraine.

"President Biden and I agreed this evening there is a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," Johnson said in a Twitter microlog.

He said all countries "are united in the face of such threats."

The prime minister also said "further incursion into Ukraine will result in far reaching damage for Russia and the world."