Washington DC - President Joe Biden on Tuesday condemned "dumb" and "shameful" comments by Donald Trump on NATO, in one of his most blistering attacks yet.

The 81-year-old 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.

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"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, which he doubled down on Monday.

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.

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"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.

House Republicans quickly made it clear, however, that the bill had no chance of passing in the chamber where they hold a small majority.