Fears of a new war in Europe resurged Thursday as U.S. President Joe Biden warned that Russia could invade Ukraine within days.

"Every indication we have is they're prepared to go into Ukraine," Biden said.

He added that the U.S. has "reason to believe" that Russia is "engaged in a false flag operation to have an excuse to go in," but did not provide details.

The White House said Biden planned to speak by phone Friday with trans-Atlantic leaders about Russia's military buildup and continued efforts at deterrence and diplomacy.

Russia is believed to have dispatched some 150,000 military forces around Ukraine's borders.

The Kremlin insists it has no plans to invade, but it has long considered Ukraine part of its sphere of influence and NATO's eastward expansion an existential threat.