A Russian attack on Ukraine is "still very much a possibility" and the human cost would be "immense", US President Joe Biden has said.
In remarks televised nationally, he said the US was ready to respond decisively to such a move.
Mr Biden said Russia had massed some 150,000 troops on Ukraine's borders.
Russia says it is pulling back some of its troops. On Wednesday, it announced that military drills in Moscow-annexed Crimea had ended.
"Units of the Southern Military District, having completed their participation in tactical exercises, are moving to their permanent deployment points," the defence ministry said in a statement, without detailing how many troops were leaving.
Footage broadcast on state television appeared to show military vehicles crossing the bridge to leave Crimea.