"We will always be there for NATO, even if they won't be there for us," he wrote on his Truth Social platform, a day after the White House said it was not ruling out military intervention to acquire Greenland.

In his post, Trump repeated his frequent assertion that many members of the North Atlantic Treaty Organization had failed to meet military spending commitments until he intervened.

"The USA was, foolishly, paying for them! I, respectfully, got them to 5% GDP, AND THEY PAY, immediately," he wrote.

But the US leader added that the NATO member countries are "all my friends."

Trump, who has cast himself as a global peacemaker, also touched on a personal grievance.

"I single-handedly ENDED 8 WARS, and Norway, a NATO Member, foolishly chose not to give me the Noble Peace Prize," he said, misspelling the name of the prestigious award that instead went to Venezuelan opposition leader Maria Corina Machado.

Trump concluded by saying that Russia and China have "zero fear of NATO" without the United States as part of the alliance.

"The only Nation that China and Russia fear and respect is the DJT REBUILT U.S.A." he said, referring to himself by his initials.

Trump's designs on the mineral-rich, self-governing Danish territory of Greenland has set off alarm bells among European NATO members, who see such a move as an existential threat to the alliance.