A Russian attack on Ukraine “remains very much a possibility” but 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.
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Its defence minister has said that some forces have now withdrawn.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.”
Mr Biden’s speech came hours after Russian President Vladimir Putin said Moscow’s security concerns should be addressed and taken seriously.
Mr Putin has always denied he is planning an incursion, and said Russia did not want another war in Europe. However, tensions have been rising since November.
Russia has deep cultural and historic ties with Ukraine, which is a former Soviet republic.
Mr Putin wants assurances that it will not join the Western Nato military alliance because he sees any expansion of it as a threat to Russia. Nato has rejected that demand.
Mr Biden said he agreed with a proposal by the Russian government to continue diplomacy.
