The head of the NATO military alliance warned on Sunday that Donald Trump was putting the safety of US troops and their allies at risk after the Republican presidential frontrunner said Russia should be able to do "whatever the hell they want" to NATO members who don't meet their defence spending targets.

"Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk," NATO Secretary-General Jens Stoltenberg said in a statement.

Speaking on Saturday at a rally in Conway, South Carolina, Trump recalled how as president he told an unidentified NATO member that he would "encourage" Russia to do as it wishes in cases of NATO allies who are "delinquent".

"You didn't pay? You're delinquent?" Trump recounted saying. "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's remarks caused deep concern in Poland, which was under Russian control in past centuries, and where anxieties are high over the war Russia is waging just across the Polish border in Ukraine.

"We have a hot war at our border," Polish Prime Minister Donald Tusk said on Sunday, voicing concerns about whether the US will show "full solidarity with other NATO countries in this confrontation that promises to last for a long time with Russia".

"We must realise that the EU cannot be an economic and civilisational giant and a dwarf when it comes to defense, because the world has changed," he argued in a town hall speech marking the start of his party's campaign for local elections this spring.

In 2014, NATO allies pledged to move toward spending two per cent of GDP on defence by 2024. According to NATO estimates in early 2023, 10 of its 30 member states at the time were close to or above the two per cent mark, while 13 were spending 1.5 per cent or less.

No country is in debt to any other, or to NATO.

In a statement, Trump senior advisor Jason Miller said that Trump would be able to more effectively force allies to increase their NATO spending compared to President Joe Biden, and that "when you don't pay your defence spending you can't be surprised that you get more war".

Stoltenberg said he expects that "regardless of who wins the presidential election, the US will remain a strong and committed NATO ally".

The German government did not officially comment on Trump's remarks, but its foreign office pointed out NATO's solidarity principle in a statement on X, formerly Twitter.

"'One for all and all for one.' This NATO creed keeps more than 950 million people safe," it said.

Trump's comments were of particular concern to NATO's frontline countries, like Poland and the Baltic states of Lithuania, Latvia and Estonia, which were either under the control of Moscow or fully incorporated into the Soviet Union during the Cold War. Fears there run especially high given Russia's invasion of Ukraine.

Polish President Andrzej Duda, who is allied with the right-wing opposition, and who was seen as friendly to Trump during his presidency, tweeted that the Polish-US alliance must be strong "regardless of who is currently in power in Poland and the USA".

He warned, "Offending half of the American political scene serves neither our economic interests nor Poland's security."

In an editorial on Sunday, German daily Frankfurter Allgemeine Zeitung also called for European nations to spend more on defence.

It said that if Trump wins the presidency again, statements like the one he made on Saturday night "will increase the risk of Putin expanding his war. Europeans can only do one thing to counter this: finally invest in their military security in line with the seriousness of the situation".

Trump's tenure, which was marked by his open admiration for Russian President Vladimir Putin, became a near-existential challenge for NATO, an organisation largely controlled by the US. The prospect that Trump might return to power remains a deep concern among allies.

Stoltenberg was praised for his diplomatic skills in keeping NATO together during the Trump years, but the former Norwegian prime minister is stepping down. His successor is likely to be announced by the time allied leaders meet in Washington for NATO's 75th anniversary summit in July.

Under NATO's mutual defence clause, Article 5 of its founding treaty, all allies commit to help any member who comes under attack. The article has only ever been activated once -- by the US in the wake of the September 11, 2001, attacks.

NATO has undertaken its biggest military buildup since the Cold War since Russia invaded Ukraine in February 2022.