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".
He told a news conference Mr Putin would pay a "serious and dear price" for invading, but indicated a minor incursion might be treated differently.
The White House later stressed any Russian military move would be met with a swift, severe response from the West.
Russia has some 100,000 troops near the border but denies planning an invasion.
President Putin has made a series of demands to the West, insisting Ukraine is never allowed to join Nato and that the defensive alliance abandons military activity in Eastern Europe.
'New security order for Europe'
Talks between the West and Russia have so far failed to reach a breakthrough, with some of Moscow's demands rejected as non-starters. US Secretary of State Antony Blinken is due to meet the Russian foreign minister in Geneva on Friday, having warned that Russia could attack Ukraine "on very short notice".
However, at his news conference on Wednesday, Mr Biden said: "There are differences in Nato as to what countries are willing to do, depending on what happens.
"If there's Russian forces crossing the border… I think that changes everything.
"What you're going to see is that Russia will be held accountable if it invades and it depends on what it does," he said. "It's one thing if it's a minor incursion, and then we end up having to fight about what to do and not do etc."
White House officials rushed to clarify the US position after Mr Biden's comments. "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," press secretary Jen Psaki said.
Hours earlier, French President Emmanuel Macron told the European Parliament that Europeans had to conduct their own dialogue with Moscow.