WASHINGTON (AFP) - The United States said on Wednesday (Feb 23) that Russian President Vladimir Putin is "as ready as he can be" for a full-scale invasion of Ukraine, with nearly 100 per cent of the necessary military forces already in place.

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

The official said that 80 per cent of the estimated 150,000-plus Russian forces along the Ukraine border are in "ready positions": spread out in attack formation within a few kilometres 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 on Tuesday the invasion is "beginning," administration officials say they do not yet have evidence that Russian forces have crossed the border into Ukraine.

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