NATO is a "sacred commitment" for the US, President Joe Biden has said as he slammed his predecessor Donald Trump for his recent remarks encouraging Russia to attack certain nations of the 74-year-old military alliance by calling the comments “dangerous” and “un-American”.

Biden said the remarks by Trump, his likely challenger in the November 5 Presidential polls, underscored the urgency of passing a USD 95 billion foreign aid package for US allies.

The bill just passed the Senate, but it faces political headwinds in the House.

At the White House on Tuesday, Biden said a failure to pass the package - which includes USD 60bn for Ukraine - would be "playing into Putin's hands".

"When America gives its word, it means something. When we make a commitment, we keep it. And NATO is a sacred commitment. Donald Trump looks at this as if it's a burden," Biden said.

"When he looks at NATO, he doesn't see the alliance that protects America and the world. He sees a protection racket. He doesn't understand that NATO is built on the fundamental principles of freedom, security, and national sovereignty," he added.

Speaking at a rally in South Carolina during the weekend, Trump said he would ask NATO allies to increase their defence spending or else encourage Russian President Vladimir Putin to invade those nations.

"You didn't pay? You're delinquent? No, I would not protect you. In fact, I would encourage them to do whatever the hell they want. You gotta pay. You gotta pay your bills," Trump said.

Trump, who is the frontrunner for the Republican Party's nomination, has called for NATO members to devote two per cent of their gross domestic product to defence.

In response, Biden said, "Because, for Trump, principles never matter. Everything is transactional. He doesn't understand that the sacred commitment we've given works for us as well. In fact, I would remind Trump and all those who would walk away from NATO, that Article 5 has only been invoked once, just once in our NATO history, and it was done to stand with America after we were attacked on 9/11." "We should never forget it. You know, our adversaries have long sought to create cracks in the alliance," he added.

Under Article 5 of NATO's mutual defence clause, all allies commit to help any member who comes under attack.

The article has only ever been invoked once -- by the US following the attacks of September 11, 2001.

Biden said, "The greatest hope of all those who wish America harm is for NATO to fall apart. And you can be sure that they all cheered when they heard Donald Trump and heard what he said. I know this. I will not walk away. I can't imagine any other president walking away. For as long as I'm president, if Putin attacks a NATO ally, the United States will defend every inch of NATO territory." The US president also described those remarks as shameful and "un-American".

"The former president (Trump) has sent a dangerous and shockingly frankly un-American signal to the world. Just a few days ago, Trump gave an invitation to Putin to invade some of our allies, NATO allies. He said if an ally didn't spend enough money on defence, he would encourage Russia to, quote, do whatever the hell they want end of quote," Biden said.

"Can you imagine a former president of the United States saying that? The whole world heard it. The worst thing is he means it. No other president in our history has ever bowed down to a Russian dictator. Let me say this as clearly as I can. I never will. For God's sake, it's dumb, it's shameful, it's dangerous, it's un-American," Biden said.

Nikki Haley, Trump's rival for the Republican presidential nomination, also slammed the former president for his comments.

"Don't take the side of a thug who kills his opponents. Don't take the side of someone who has gone in and invaded a country, and half a million people have died or been wounded because of Putin. Don't take the side of someone who continues to lie. I dealt with Russia every day. The last thing we ever want to do is side with Russia," she told CBS in an interview on Sunday.

"We do want NATO allies to pull their weight but there are ways you can do that without sitting there and telling Russia, 'Have your way with these countries'. If you notice, Russia has never invaded a NATO country. They've invaded Georgia, Moldova and Ukraine. They are actually very intimidated by NATO. NATO allows us to prevent war," Haley had said.

White House Spokesperson Andrew Bates said Biden has restored alliances and made the US stronger because he knows every commander-in-chief's first responsibility is to keep the American people safe and hold true to the values that unite them.

He said: "Thanks to President Biden's experienced leadership, NATO is now the largest and most vital it has ever been. Encouraging invasions of our closest allies by murderous regimes is appalling and unhinged -- and it endangers American national security, global stability, and our economy at home." Bates added, "Rather than calling for wars and promoting deranged chaos, President Biden will continue to bolster American leadership and stand up for our national security interests -- not against them." In a separate statement, Biden said the US has stood shoulder to shoulder with allies under his administration to build a NATO that is bigger and stronger and stands in defence of democracy against Putin's "authoritarian aggression".

"America's leadership on the world stage and support for our allies is critical to keeping the American people safe here at home. 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," he said.

Senator Susan Collins -- vice-chairman of the Senate Appropriations Committee and Subcommittee on Defense -- said if Russia prevails in Ukraine, Putin will be in a position to threaten the US' NATO allies all along that country's borders, including Poland, and Finland -- one of the military alliance's newest members.

The Trump Campaign, however, has stood in support of the former president and asserted that his administration made NATO stronger.

"President Trump knew that NATO was broke or busted until he took over because people weren't paying their dues," Senator Marco Rubio told CNN on Sunday.

Make America Great Again CEO Taylor Budowich said, "He used leverage to get people to step up to the plate and become more active in NATO … He's been president before, I know exactly what he's done and (what he) will do with the NATO alliance." "President Trump secured commitments from NATO member nations to increase their defence contributions by over USD 100 billion. NATO Secretary General Jens Stoltenberg praised President Trump for his 'strong leadership', for his 'commitment to the alliance' and thanked him for spurring increased defence investment. Foreseeing the issue, President Trump sanctioned the Russian Nord Stream 2 pipeline and even warned Germany and other European nations against relying on Russian energy," Budowich said.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.