Both when campaigning and once in the White House, Trump often sought to publicly undermine NATO, the US-led military alliance at the heart of Western geopolitical power since World War II, but which he described as a group of countries living off US largesse.

However, his comments late Thursday to an event hosted by the right-wing Heritage Foundation were unusually stark, boasting that the media had failed to notice his threat.

Although NATO's Article 5 clause requires members come to the defense of any part of the alliance under attack, Trump said he suggested at a summit that the treaty wouldn't be honored, because "everybody was delinquent. They didn't pay."

"They asked me, one of the presidents..., 'does that mean you won't protect us in case of we don't pay, you won't protect us from Russia?'... I said, 'that's exactly what that means.'"

Trump, who has frequently described himself as a master negotiator, indicated to laughter at the event that he was bluffing in order to extract concessions.

"Now if I said, 'No, I don't mean that,' then why would they pay? So somebody had to say it," he said in the comments broadcast by the conservative media outlet RSBN.

Trump went on to say he was "amazed it didn't get out. The fake news didn't pull it out."

And according to the Republican, his threats worked because "the money started to flow in."

Trump regularly tore into NATO and other long-established US alliances, arguing that the United States spent too much money defending countries that he said were more like fierce rivals than partners.

"We're defending Europe. At the same time they rip us off on trade, very, very seriously rip us off," he said in Thursday's comments, reprising one of his favorite lines of attack on the transatlantic allies.

Trump was also frequently accused of having a deferential attitude to Kremlin leader Vladimir Putin, whose invasion of non-NATO member Ukraine in February has triggered a massive response from the Washington and its European allies.

In 2020, Trump was also impeached for linking delivery of US military aid for Ukraine to a demand that Ukrainian President Volodymyr Zelensky try to dig up on political dirt on then presidential candidate Joe Biden.