A Russian attack on Ukraine "remains very much a possibility" but the human cost would be "immense", US President Joe Biden has said, BBC reports.
In remarks televised nationally, he said the US was ready to respond decisively to such a move.
Russia has amassed more than 100,000 troops near Ukraine's border, prompting fears of an invasion.
Its defence minister has said that some forces have now withdrawn. Mr Biden warned that this had not been verified.
"[Russian forces leaving] would be good, but we have not yet verified that. We have not yet verified the Russian military units are returning to their home bases," the US president said.
"Indeed, our analysts indicate that they remain very much in a threatening position."