NATO partners who don't fulfil their financial obligations would not get protection from Russia if Donald Trump were to re-take the White House in November, statements from Trump showed.
Trump, campaigning in South Carolina ahead of the GOP primary on February 24, said he had told NATO members that they needed to pay up.
"I said, 'Everybody's gonna pay.' They said, 'Well, if we don't pay, are you still going to protect us?' I said, 'Absolutely not.' They couldn't believe the answer."
And, he said, he would "encourge them (Russia) to do whatever the hell they want. You gotta pay." Trump claimed that "NATO was busted until I came along."
US President Joe Biden's White House condemned Trump immediately.
"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, said spokesman Andrew Bates said in a statement on Saturday.
Biden has "restored our alliances," Bates said, adding that NATO is the "largest and most vital it has ever been."
During a recent visit to Washington, NATO Secretary General Jens Stoltenberg countered fears that a Trump win could jeopardize NATO's future. He had worked with Trump for four years and listened to him carefully, Stoltenberg told the US broadcaster CNN at the end of January.
Trump's main criticism - that alliance partners spend too little on NATO - has been heard. They had significantly increased their defence spending in recent years.
Under Article 5 of the NATO treaty, an attack on one NATO member is considered as an attack on all and the defence alliance "will take" actions needed to defend the attacked ally.
The clause has only been enacted one - after the September 11 terrorist attacks in the US in 2001.