Joe Biden has warned that the US will 'respond forcefully' if Russia targets Americans in Ukraine.
The President urged Russia to step back from the brink during a televised speech about a feared Russian invasion of Ukraine.
He said an attack is still 'distinctly possible', despite a Russian defence ministry spokesman saying some troops had pulled back from the border.
It comes after US spies warned that Russia could invade Ukraine on Wednesday, with one source claiming the time of the attack was set for 3am.
Biden said: "If Russia targets Americans in Ukraine we will respond forcefully.
"If Russia attacks the United States or our allies through asymmetric means like disruptive cyber attacks against our companies or critical infrastructure we are prepared to respond.
"The Russian defence minister reported today that some military units are leaving their positions near Ukraine.
"That would be good but we have not yet verified that. More than 150,000 troops remain along Ukraine's border. An invasion remains distinctly possible.
"We are ready to respond decisively to a Russian attack on Ukraine, which remains very much a possibility"
The President also made a direct appeal to the people of Russia, telling them 'you are not our enemy'.
Mr Biden said the US and NATO are not a threat to Russia and promised America did not have missiles in Ukraine.
He also ruled out placing missiles in the country and said he did not wish to destabilise Russia.
American intelligence agencies believe Kremlin chiefs will order an attack on Ukraine at 3am local time tomorrow.
They could target Kyiv’s military and government command and control centres with a barrage of air strikes before tanks roll over the border.
At the same time Russian amphibious warships could storm Ukraine’s southern coastline.
It comes as Foreign Secretary Liz Truss warned a Russian invasion 'would not stop at Ukraine'.
Meanwhile, two of Ukraine's banks were targeted in a cyber attack today, according to the country's security experts.
The country's largest state-owned banks PrivatBank and Oschadbank have been affected by the strike, with the country's Ministry of Defence website also reportedly crashing.
Websites for Oschadbank and Ukraine's Ministry of Defence were down when The Mirror tried to access them this afternoon.
The Centre for Strategic Communications and Information Security of Ukraine, known as the Stratcom Centre, confirmed that the PrivatBank app had gone down for some users.
The Stratcom Centre, which is part of the Ukrainian Government and counters online threats, wrote on Facebook: "During the last hours 'PrivatBank' is under a massive DdoS-attack.
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"Privat24 users report problems with payments and in general with the operation of the app. Some users are completely unable to access Privat24, others do not reflect balance and recent transactions.
"There is no threat to investors' funds. Cyberattack is exclusively related to the Privat24 app. The rest of the financial transactions are carried out in a normal manner.
"In the work of 'Oschadbank' there are also errors, internet banking does not work.
"Also suffered attacks on the sites of the Ministry of Defence and the Armed Forces. It is no exception that the aggressor has succeeded in the tactics of small hoods, because most of his aggressive plans are not working."
