US President Joe Biden and British Prime Minister Boris Johnson have warned an attack on Ukraine could come within days. They discussed the crisis on Monday over the phone.

Biden and Johnson agreed there remains a "crucial window for diplomacy." They said any incursion would result in "a protracted crisis" with severe economic consequences for Russia.

The allies agreed Europeans need to reduce their dependence on Russia for energy. Johnson cited a gas pipeline to Germany. The Nord Stream 2 line has been built but is not yet in operation.

Russia is one of the world's largest providers of natural gas. Johnson and Biden agreed Europeans need to look for alternative sources of energy.