Trump, appearing to recount a meeting with NATO leaders during a political rally in South Carolina on Saturday, quoted the president of “a big country” that he did not name as asking, “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?’ He said: ‘Yes, let’s say that happened.’ No I would not protect you. In fact I would encourage them to do whatever the hell they want. You gotta pay.”

White House spokesman Andrew Bates, asked about Trump’s comments, said: “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.”

The NATO treaty contains a provision that guarantees mutual defence of member states if one is attacked.

Trump, frontrunner for the Republican presidential nomination, was a fierce critic of the North Atlantic Treaty Organisation when he was president, repeatedly threatening to pull out of the alliance.

He cut defence funding to NATO and frequently complained the US was paying more than its fair share.

Bates said President Joe Biden, a Democrat seeking re-election in the November election, had restored US alliances after taking office in 2021, ensuring NATO was now “the largest and most vital it has ever been”.

“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,” he said in a statement issued late on Saturday.

With Trump leading Biden in some polls, European allies worry a Trump victory in November could jeopardise the US commitment to the alliance, but NATO Secretary General Jens Stoltenberg last month said he did not think a second Trump presidency would jeopardise US membership.

Stoltenberg, who has been pushing member states to boost defence spending, said European allies were increasing their military contributions and “moving in the right direction”.

Trump has continued to hammer the transatlantic alliance, telling a campaign rally last month that he did not believe NATO countries would support the US if it were attacked.

On Russia’s war in Ukraine, Trump has called for de-escalation and complained about the billions spent so far, although he has put forward few tangible policy proposals.

Since Moscow’s full-scale invasion in February 2022, US aid to Ukraine has totalled around $US75 billion ($A115 billion), Stoltenberg said, while other NATO members and partner states combined have provided more than $US100 billion ($A153 billion).