German Chancellor Olaf Scholz is expected in Kyiv on Monday before moving on to Moscow in his efforts to head off the "critical" threat of a Russian invasion that could spark the worst political crisis in Europe since the Cold War.
Russian President Vladimir Putin has deployed more than 100,000 soldiers in a high-stakes standoff with the West over NATO's expansion into countries once under the Kremlin's control.
The western allies have remained united and defiant in the face of Putin's demands for binding security guarantees that would see NATO roll back its forces and rule out Ukraine's potential membership of the defensive alliance.
But US intelligence officials worry that weeks of crisis talks have given Russia the time to prepare a major offensive -- should Putin make the ultimate decision to attack Ukraine.
Washington reaffirmed its warning on Sunday that Russia was now ready to strike at "any moment".
US President Joe Biden briefed Ukrainian leader Volodymyr Zelensky on Sunday about his hour-long phone call with Putin the previous day. Biden's talks with Putin broke no new ground, the White House said.
US officials said Biden and Zelensky had "agreed on the importance of continuing to pursue diplomacy and deterrence" in their call.
German call for sanctions
Germany's Scholtz was firm in his resolve to support Ukraine and hit Russia "immediately" with punishing sanctions should Moscow invade.
"In the event of a military aggression against Ukraine that threatens its territorial integrity and sovereignty, that will lead to tough sanctions that we have carefully prepared and which we can immediately put into force," Scholz said on the eve of his departure.
"We assess the situation as very critical, very dangerous," a German government source added.
Germany's close business relations with Moscow and the country's heavy reliance on Russian natural gas imports have been a source of lingering concern for Kyiv's pro-western leaders as well as Biden's team.
Scholz has warned Russia it should "not underestimate our unity and determination" but also hedged against unequivocally backing Biden's pledge to "bring an end" to Russia's new Nord Stream 2 gas link to Germany.
Johnson enters the fray
Scholz's visit to Moscow on Tuesday will be clouded by a spat involving the tit-for-tat closures of the German-language channel of Russia's RT network and the Moscow bureau of Germany's Deutsche Welle.
British Prime Minister Boris Johnson will also "travel to Europe" to address the crisis towards end of the week as part of the European push for peace, his office said on Sunday.
