Joe Biden has said he would consider imposing personal sanctions on Vladimir Putin if Russia invades neighbouring Ukraine.
The US president also said there would be “enormous consequences” for the world if President Putin’s troops push into the former Soviet state, that lies on its south-western border.
Some 8,500 US troops have been put on heightened alert to go to eastern Europe to bolster NATO forces.
The soldiers are on standby and Mr Biden said some could be deployed quickly if needed, while a Pentagon official said more could be added to the current figure.
However the president vowed no American troops would be sent specifically to Ukraine.
Mr Biden was asked by the media if he would consider sanctioning Mr Putin personally if there was an offensive. So far, most US sanctions have hit Mr Putin’s allies and Russian trade.
The US president said: “Yes. I would see that.”
‘Largest invasion since WWII’
This would be a rare move as the US normally does not impose sanctions on the leaders of foreign countries.
Mr Biden also said if Russia invades Ukraine with all of its forces, it would be the “largest invasion since World War Two” and would “change the world”.
Explainer: What’s going on at Russia-Ukraine border – and where is Moscow hiding its tanks and missiles?
Meanwhile, Boris Johnson has said Britain is prepared to send troops to protect NATO allies in Europe if there is an invasion and warned Mr Putin his forces would face “ferocious” Ukrainian resistance.
The prime minister also said the UK and its allies are ready to bring in “heavy economic sanctions” against Russia and he expressed concerns that a major military assault would lead to “bloodshed comparable to the first war in Chechnya or Bosnia”.
And in a televised address on Tuesday evening, Ukrainian President Volodymyr Zelenskiy urged his citizens to stay calm and said work was taking place for a meeting between him and the leaders of Russia, Germany and France.
Read more: Inside tank base that provides Ukraine’s best hope of defence against Russian invasion
Tensions remain high after NATO said on Monday it was putting forces on standby and sending more ships and fighter jets to eastern Europe in response to Russia’s troop build-up near the Ukraine border.
NATO currently has about 4,000 troops in multinational battalions in Estonia, Lithuania, Latvia and Poland, backed by tanks, air defences and intelligence and surveillance units.
Russia watching with ‘great concern’
Moscow has denied it is planning an assault and said it was watching with “great concern”.
It has massed about 100,000 troops near Ukraine in recent weeks and is holding military drills at multiple locations in Russia and neighbouring Belarus, which also borders Ukraine.
This has led the United States and its NATO allies to rush to prepare for a possible war.
Russia is demanding security guarantees from the West, including that NATO must rule out allowing Ukraine to join the pact as it views the former Soviet republic as a buffer between itself and NATO countries.