The 81-year-old Democrat accused his predecessor of bowing to Russian President Vladimir Putin, after Trump said he would encourage Moscow to attack NATO members who failed to meet financial commitments.

"For God's sake, it's dumb, it's shameful, it's dangerous, it's un-American," Biden said in a televised address from the White House to urge the House of Representatives to pass vital war aid for Ukraine.

"Can you imagine a former president of the United States saying that? The whole world heard it. And the worst thing is he means it," he added.

"No other president in our history has ever bowed down to a Russian dictator. Well, let me say this as clearly as I can: I never will."

Washington's allies reacted with alarm after Trump made his most extreme broadside yet against the US-led military alliance -- even by his standards of long-term NATO-bashing.

Biden used Trump's comments to fuel his election attack line against Trump -- who was impeached twice as president and now faces a series of criminal trials -- as a threat to democracy.

In his remarks from the state dining room at the White House, he accused the real estate tycoon of acting like an organized crime boss when it came to the alliance.

"When he looks at NATO, he doesn't see the alliance that protects America and the world. He sees a protection racket," Biden said.

Biden added that if Trump's allies in the House fail to follow the lead of the Senate and pass a bill with billions of dollars in military assistance for Ukraine, then they will be playing into Putin's hands.

Trump made the comments at a campaign rally in South Carolina on Saturday, describing what he said was a conversation with a fellow head of state at an unspecified NATO meeting.

"One of the presidents of a big country stood up and said, 'Well, sir, if we don't pay, and we're attacked by Russia, will you protect us?' I said, 'You didn't pay, you're delinquent? No, I would not protect you,'" Trump told his supporters.

"In fact, I would encourage them to do whatever the hell they want."

A defiant Trump later defended his comments, saying he had made NATO "strong" by making allies meet defense spending targets when he was in office from 2017-2021.

Trump has a long history of praising the Kremlin leader, for example calling him a "genius" and more credible than US intelligence.

He has also said that as president, he could settle the Ukraine war within 24 hours.