Joe Biden has warned Vladimir Putin he faces personal sanctions if Russia invades Ukraine.
The threat came as America, the UK and other European allies continued talks to deploy thousands of forces to NATO countries in preparation of Moscow calling their troops forward.
Each country would reportedly send 1,000 additional soldiers with Romania, Bulgaria and Hungary among the countries expected to receive them.
The US President said there would be "enormous consequences" for the world if Russia invades Ukraine, which sits on its south-western border.
When Mr Biden was asked whether he could see himself imposing sanctions on Putin personally in the event of an invasion, he replied “Yes”.
Russia has accused the US and others of "escalating tensions" over the issue and denies planning to enter Ukraine despite its leader deploying an estimated 100,000 soldiers near the border.
The Kremlin has said it sees the Western military alliance Nato as a security threat, and is demanding legal guarantees it will not add new members further east, including neighbouring Ukraine.
But the US has said the issue at stake is Russian aggression, not Nato expansion.
Mr Biden said a move across Ukraine's border by Moscow would have "enormous consequences worldwide”.
He said: "If he were to move in with all those forces, it would be the largest invasion since World War II. It would change the world.”
Asked about what would lead him to deploy the troops staging nearby, Biden said that depends on "what Putin does or doesn't do" but he repeated American forces would not move into Ukraine.
He added: "I may be moving some of those troops in the nearer term, just because it takes time," Biden said, adding it's not to be "provocative" but to reassure NATO allies who have reasons for concern.
"We have no intention of putting American forces, or NATO forces, in Ukraine. But we, as I said, they’re gonna be serious economic consequences if he [Putin] moves.”
Today Foreign Minister Liz Truss echoed the President's words, telling the BBC's Today programme the government has "ruled nothing out”.
She said the UK would also consider sanctions against the Russian leader adding they are looking to "toughen up" its restrictions on people, banks and businesses.
"What is important is that all of our allies do the same. It is by collective action. By showing Putin we're united, we will help deter a Russian incursion," she said.
Ms Truss spokes after Prime Minister Boris Johnson raised the issue on Tuesday of banning Russia from the Swift international payments system.
Senior Russian officials said the move would mean Europe would not be able to pay for and receive Russian products.
On Monday the US put 8,500 troops in the US on heightened alert in case a NATO Response Force is called up and American forces are needed quickly.
However, the vast majority of these forces are intended for activity supported by the full NATO alliance.
