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.