U.S. President Joe Biden and top western officials criticized former president Donald Trump on Sunday after he suggested the United States might not protect NATO allies who aren't spending enough on defence from a potential Russian invasion.

"America’s leadership on the world stage and support for our allies is critical to keeping the American people safe here at home," said Biden in a statement.

"If my opponent, Donald Trump, is able to regain power, he is making it clear as day that he will abandon our NATO allies if Russia attacks and allow Russia to 'do whatever the hell they want' with them."

“Tonight’s results leave the American people with a clear choice: Are we going to keep moving forward or will we allow Donald Trump to drag us backwards into the chaos, division, and darkness that defined his term in office?

“Four years ago, I ran because of the existential threat Donald Trump posed to the America we all believe in. Since then, we’ve made enormous progress: 15 million jobs, wages rising faster than inflation, taking on Big Pharma and the gun lobby — and winning. But we have more to do.

“If Donald Trump returns to the White House, all of this progress is at risk. He is driven by grievance and grift, focused on his own revenge and retribution, not the American people. He is determined to destroy our democracy, rip away fundamental freedoms like the ability for women to make their own health care decisions, and pass another round of billions of dollars in tax cuts for the wealthy — and he’ll do or say anything to put himself in power.

“Today, millions of voters across the country made their voices heard — showing that they are ready to fight back against Donald Trump’s extreme plan to take us backwards.

“My message to the country is this: Every generation of Americans will face a moment when it has to defend democracy. Stand up for our personal freedom. Stand up for the right to vote and our civil rights. To every Democrat, Republican, and independent who believes in a free and fair America: This is our moment. This is our fight. Together, we will win.”