Donald Trump has again expressed interest in buying Greenland ...

President-elect Donald Trump has refused to rule out using military force to seize control of the Panama Canal and Greenland, declaring US control of both to be vital to American national security.

Less than two weeks before he took office, and amid a Donald Trump Jr visit to Greenland, Trump left open the use of the American military to secure both territories.

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

When asked if he would rule out the use of the military, Trump said: "I'm not going to commit to that."

"It might be that you'll have to do something. The Panama Canal is vital to our country," he said.

Here's why Trump's so interested in the world's largest island.

Why does Trump want to buy Greenland?

Trump has floated buying the island for years.

Greenland has strategic significance for the US military and its ballistic missile early warning system, since the shortest route from Europe to North America runs via the Arctic island.

He told reporters: "We need Greenland for national security purposes."

Greenland also boasts mineral, oil and natural gas wealth, but development has been slow.

Could Trump actually buy it?

The governments of both Denmark and Greenland have come out strongly against the idea the Arctic nation can be bought.

"We are not for sale and will never be for sale," Greenland Prime Minister Mute Egede wrote in a Facebook post in late December.

On Tuesday, Egede said it was up to its people to decide their future.

"While others, including Danes and Americans, are entitled to their opinions, we should not be caught up in the hysteria and let external pressures distract us from our path," he said.

Who owns Greenland?

Greenland is part of Denmark.

Greenland says it is not for sale

Greenland tells Donald Trump the country is not for sale, responding to the US president-elect's comments about taking control of the territory.

It became a formal territory of the Nordic Kingdom in 1953 and is subject to the Danish constitution, meaning that any change to its legal status would require a constitutional amendment.

In 2009, the island was granted broad self-governing autonomy, including the right to declare independence from Denmark through a referendum.

It is a longtime US ally and a founding member of NATO.

If Greenland becomes independent, it could choose to become associated with the US.

But Aaja Chemnitz, a Greenlandic member of the Danish parliament, said the idea of a US takeover should be firmly rejected.

"I don't want to be a pawn in Trump's hot dreams of expanding his empire to include our country," she wrote.

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

"People really don't even know if Denmark has any legal right, but if they do, they should give it up, because we need it for national security," he said.

Denmark and Greenland respond to military comments

Addressing Trump's comments in an interview with Danish broadcaster TV2, Danish Prime Minister Mette Frederiksen said the US was Denmark's "most important and closest ally".

She added that she did not believe Washington would use military or economic power to secure control of Greenland.

Ms Frederiksen said she welcomed the US for taking a greater interest in the Arctic region, but it would "have to be done in a way" that was "respectful of the Greenlandic people".

"[And] 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," she said.

Earlier, Trump posted a video of his private plane landing in the Arctic territory's capital, Nuuk, showing 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 was visiting "as a private individual" and it was not an official visit, adding that Greenland representatives would not meet with him.

Who owns the Panama Canal?

The Panama Canal is a major waterway connecting the Atlantic and Pacific oceans.

Trump threatens to retake control of Panama Canal

President-elect Donald Trump muses about regaining US control over the Panama Canal, accusing Panama of charging excessive fees for use of the waterway. 

The government of Panama has owned and operated the canal for 25 years through the Panama Canal Authority.

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

In response to Trump's comments, Panama said the sovereignty of its interoceanic canal was non-negotiable.

"The sovereignty of our canal is not negotiable and is part of our history of struggle," Foreign Minister Javier Martinez-Acha said, noting that President Jose Raul Mulino had made his stance clear.

Renaming the Gulf of Mexico

Trump has also floated having Canada join the US, but has said he would rely on "economic force" rather than the military to make that happen.

Promising a "golden age of America," Trump also said he would move to try to rename the Gulf of Mexico as the "Gulf of America", saying it has a "beautiful ring to it."

Trump also used his press conference to complain that US President Joe Biden was undermining his transition to power, a day after the incumbent moved to ban offshore energy drilling in most federal waters.

Mr Biden used his authority to protect offshore areas along the east and west coasts in the US, the eastern Gulf of Mexico and portions of Alaska's Northern Bering Sea from future oil and natural gas leasing.

But, Trump said he was going to "put it back on day one" and pledged to take it to the courts if necessary.

AP/Reuters