US President-elect Donald Trump on Tuesday said he would not rule out the use of military force to seize control of the Panama Canal and Greenland, as he declared US control of both to be vital to American national security.

Speaking to reporters less than two weeks before he takes office on January 20 and as a delegation of aides and advisers, that includes Donald Trump Jr., is in Greenland, Trump left open the use of the American military to secure both territories.

Trump's intention marks a rejection of decades of US policy that has prioritised self-determination over territorial expansion.

“I’m not going to commit to that," Trump said, when asked if he would rule out the use of the military.

"It might be that you’ll have to do something. The Panama Canal is vital to our country.”

He added, “We need Greenland for national security purposes."

Greenland, home to a large US military base, is an autonomous territory of Denmark, a longtime US ally and a founding member of Nato.

Trump cast doubts on the legitimacy of Denmark's claim to Greenland.

The Panama Canal has been solely controlled by the eponymous country for more than 25 years.

The US returned the Panama Canal Zone to the country in 1979 and ended its joint partnership in controlling the strategic waterway in 1999.

Addressing Trump's comments in an interview with Danish broadcaster TV2, Prime Minister Mette Frederiksen called the United States Denmark’s “most important and closest ally,” and that she did not believe that the United States would use military or economic power to secure control over Greenland.

Frederiksen repeated that she welcomed the United States taking a greater interest in the Arctic region, but that it would “have to be done in a way that is respectful of the Greenlandic people,” she said.

“At the same time, it must be done in a way that allows Denmark and the United States to still cooperate in, among other things, Nato,” Frederiksen said.

Earlier, Trump posted a video of his private plane landing in Nuuk, the Arctic territory’s capital, in a landscape of snow-capped peaks and fjords.

“Don Jr. and my Reps landing in Greenland,” Trump wrote.

“The reception has been great. They, and the Free World, need safety, security, strength, and PEACE! This is a deal that must happen. MAGA. MAKE GREENLAND GREAT AGAIN!”

In a statement, Greenland’s government said Donald Trump Jr.’s visit was taking place “as a private individual” and not as an official visit, and Greenlandic representatives would not meet with him.

Panamanian Foreign Minister Javier Martínez-Acha said his government hasn’t had formal contact with Trump or representatives of the incoming administration, but reiterated previous comments from the country’s president, José Raúl Mulino, who said last month that the canal will remain in Panamanian hands.

“The sovereignty of our canal is not negotiable and is part of our history of struggle and an irreversible conquest,” Martínez-Acha said.

Trump, a Republican, has also floated having Canada join the United States as the 51st state. He said Tuesday that he would not use military force to invade the country, which is home to more than 40 million people and is a founding Nato partner.

Instead, he said, he would would rely on “economic force" as he cast the US trade deficit with Canada — a natural resource-rich nation that provides the US with commodities like crude oil and petroleum — as a subsidy that would be coming to an end.

Canadian leaders fired back after earlier dismissing Trump's rhetoric as a joke .

“President-elect Trump’s comments show a complete lack of understanding of what makes Canada a strong country. Our economy is strong. Our people are strong. We will never back down in the face of threats,” Canadian Foreign Minister Mélanie Joly said in a post on X. (AP)