US President Joe Biden warned late Thursday more Russian troops moving toward the border with Ukraine, indicating an invasion could begin withing days.
The warning came after NATO allies voiced concern that Moscow was seeking a pretext for war, with some 150,000 Russian troops posted around the border with Ukraine.
Some 60% of Russia's ground forces are close to the frontier.
"Every indication we have is they're prepared to go into Ukraine, attack Ukraine," Biden told reporters at the White House.
Biden said the US had "reason to believe" that Russia is "engaged in a false flag operation to have an excuse to go in," but he did not provide details.
The Kremlin - has long considered Ukraine part of its sphere of influence - has said it does not plan to invade.
It considers NATO's expansion eastwatd an existential threat, and is demanding that the alliance promises it will never allow Ukraine to join.
Germany urges Russia to change tack
As Western leaders arrived in Germany for the Munich Security Conference (MSC), German Foreign Minister Annalena Baerbock said Moscow needed to show "serious steps towards de-escalation."
She accused Russia — in its call for NATO to close the door to Kyiv — of making demands reminiscent of the Cold War.
"With an unprecedented deployment of troops on the border with Ukraine and Cold War demands, Russia is challenging fundamental principles of the European peace order," Baerbock said in a statement.
This year's MSC is overshadowed by the fear of a large-scale war in Ukraine, with a meeting of the G7 foreign ministers on the crisis scheduled on the sidelines. Russia decided not to send a delegation this year.
Ahead of the opening ceremony, Baerbock said, "It is a loss that Russia is not taking advantage of this opportunity."
US Vice President Kamala Harris and Blinken, the most senior US officials present at this year's event, arrived in Germany late Thursday.
EU 'ready to mobilize' humanitarian aid
The EU Commission vice-president, Margaritis Schinas, told German newspaper Die Welt in comments published Friday that the EU could expect anywhere between 20,000 and over one million refugees in the event of military escalation in Ukraine.
He also said that roughly 20,000 EU citizens remain in Ukraine at present. In the event of a dramatic escalation of hostilities, they would likely need assistance to evacuate the country.
The EU is "ready to mobilize significant humanitarian aid and help with civilian protection," he said.
