US President Joe Biden has said he thinks his Russian counterpart Vladimir Putin will "move in" on Ukraine, but does not want "full-blown war".
Asked at a news conference about the threat of a Russian invasion, he said: "My guess is he will move in, he has to do something."
But he warned that the Russian leader would pay a "serious and dear price" for "testing" the West.
Moscow denies planning to attack or invade but has built up its forces.
It is estimated to have about 100,000 troops close to Ukraine's borders.
White House officials later issued a statement clarifying the US position, after some reporters at the news conference questioned Mr Biden about whether the US would allow a small incursion into Ukraine following his comments.
In a statement issued on Wednesday night, White House press secretary Jen Psaki said: "If any Russian military forces move across the Ukrainian border, that's a renewed invasion, and it will be met with a swift, severe, and united response from the United States and our Allies."
Russia has made a raft of demands to Western governments, including that Ukraine should never join Nato and that the defensive alliance's military activities should be limited in member states including Poland.
Talks between the West and Russia last week failed to reach a breakthrough, with some of Moscow's demands rejected as non-starters.
Russia annexed Ukraine's Crimean peninsula in 2014 after the overthrow of a pro-Russian government in Ukraine. It has supported pro-Russian rebels who control parts of eastern Ukraine in a bloody war with government forces.
There are fears that the conflict, which has cost at least 13,000 lives and caused at least two million people to flee their homes, may reignite, with the Russian military openly intervening.
Putin will 'test the West'
Asked for his views on Mr Putin's intentions, Mr Biden said: "Do I think he'll test the West? Test the United States and Nato as significantly as he can? Yes, I think he will but I think he'll pay a serious and dear price for it."
"He's never seen sanctions like the ones I promised will be imposed if he moves," Mr Biden said, adding that the exact level of punishment would depend on the scale of any invasion.
Asked how concerned he was about the possibility of a conflict in Ukraine sucking in neighbouring Nato states, he said: "The only war that is worse than one that's intended is when it's unintended and what I'm concerned about is that this could get out of hand.
"I am hoping that Vladimir Putin understands that he is, short of a full-blown nuclear war, he is not in a very good position to dominate the world. And so I don't think he thinks that, but it is a concern."
He also said it was "very important" Nato allies stay on the same page in the face of a potential Russian attack.
Mr Biden said that the US remains open for discussions with Russia and that a summit with Mr Putin is still a "possibility".
US Secretary of State Antony Blinken is to meet Russian Foreign Minister Sergei Lavrov in Geneva on Friday, after a visit to Ukraine and talks with European allies in Berlin.
Speaking in Kyiv, Mr Blinken said Russia could attack Ukraine "on very short notice" and warned again of tough sanctions if it did.
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