Western countries are winding down their diplomatic missions and urging their citizens to leave immediately after a frantic week of diplomacy failed to calm one of the most explosive standoffs since the Cold War.

US President Joe Biden briefed Ukrainian leader Volodymyr Zelensky about his hour-long phone conversation with Russia’s Vladimir Putin.

The White House said there had been no breakthrough during Saturday’s talks with the Kremlin chief.

It said Biden and Zelensky “agreed on the importance of continuing to pursue diplomacy and deterrence” in their call.

US national security advisor Jake Sullivan issued a grim assessment that an invasion that could begin begin “any day now” would likely start with “a significant barrage of missiles and bomb attacks”.

Western leaders are pushing back against Putin’s demands that the US-led NATO alliance withdraw from eastern Europe and never expand into Ukraine.