"We assess today that he is near 100 percent of all the forces that we anticipated that he would move in. He is dang near at 100 percent," a US defense official told reporters on grounds of anonymity.

The official said that 80 percent of the estimated 150,000-plus Russian forces along the Ukraine border are in "ready positions": spread out in attack formation within a few kilometers of the frontier.

"He is as ready as he can be," the official said. "Whether they actually go or not is really up to Mr. Putin ... They could go at any hour now."

While US President Joe Biden said Tuesday the invasion is "beginning," administration officials say that do not yet have evidence that Russian forces have crossed the border into Ukraine.

"We still cannot confirm that Russian military forces have moved in the Donbas areas," the defense official said.