US president Joe Biden has said that any Russian troop movements across Ukraine's border would constitute an invasion and that Moscow would "pay a heavy price" for such an action.
Mr Biden's warning is the latest White House effort to clear up comments the American leader made when he suggested that a "minor incursion" by Russia into Ukrainian territory could result in a more measured response by the United States and allies.
Facing an avalanche of criticism from Republicans and Ukrainian officials that his comments had invited limited military action by Russian president Vladimir Putin, the US leader sought to clarify his remarks at the start of a meeting at the White House focused on domestic policy.
"I've been absolutely clear with President Putin," Mr Biden said.
"He has no misunderstanding: Any, any assembled Russian units move across the Ukrainian border, that is an invasion."
Mr Biden added that an invasion would be met with a "severe and co-ordinated economic response".
His comments came as US secretary of state Antony Blinken prepared to meet Russian foreign minister Sergey Lavrov in Geneva in a high-stakes bid to ease tensions that appears likely to fail.
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Mr Biden said the US is preparing for Russia to take action that falls outside the parameters of conventional warfare.
"Russia has a long history of using measures other than overt military action to carry out aggression — paramilitary tactics, so-called grey zone attacks and actions by Russian soldiers not wearing Russian uniforms," he said.
On Wednesday, Mr Biden said he thinks Moscow will invade and warned Mr Putin that Russia would pay a "dear price" in lives lost and a possible cut-off from the global banking system if it does.
But Mr Biden also prompted consternation among allies by saying the response to a Russian invasion "depends on what it does".
"It's one thing if it's a minor incursion and then we end up having a fight about what to do and not do, et cetera," he said.
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Ukrainian president Volodymyr Zelenskyy was among those expressing concern and said he wanted to "remind the great powers that there are no minor incursions and small nations".
"Just as there are no minor casualties and little grief from the loss of loved ones," he tweeted.
Before travelling to Geneva, Mr Blinken warned in Berlin that there would be a "swift, severe" response from the United States and its allies if Russia sent any military forces into Ukraine.
Russia denies it is planning an invasion and, in turn, accused the West of plotting "provocations" in Ukraine, citing the delivery of weapons to the country by British military transports in recent days.
Russia wants binding security guarantees, including a permanent prohibition on Ukrainian membership in NATO, to which Kyiv aspires, and the removal of most of the US and allied military presence in eastern Europe.