Davos, Switzerland - President Donald Trump demanded immediate talks on acquiring Greenland in a hardline speech at Davos on Wednesday, but said that while the US possessed "unstoppable force," it would not use it to take the Arctic island from Denmark.

Trump's quest to take control of Greenland, which he said was needed for national and international security, dominated his address to the World Economic Forum as he returned for the first time in six years.

"It's the United States alone that can protect this giant mass of land, this giant piece of ice," Trump told a packed audience of global political and business leaders in the Swiss ski resort who queued for more than an hour to listen.

"That's the reason I'm seeking immediate negotiations to once again discuss the acquisition of Greenland by the United States."

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Denmark has "a choice," he said. "You can say yes, and we will be very appreciative. Or you can say no and we will remember. A strong and secure America means a strong NATO."

But Trump appeared to rule out the threat of military action against fellow NATO member Denmark – which he branded "ungrateful" – that has threatened to tear the transatlantic alliance apart.

"We probably won't get anything unless I decide to use excessive strength and force where we would be, frankly, unstoppable, but I won't do that," Trump said, adding that it was probably the "biggest statement I made."

"I don't have to use force. I don't want to use force. I won't use force."

But Trump lambasted Europe on a number of fronts from Greenland to tariffs and the economy, saying it was "not heading in the right direction."

Trump's claims over mineral-rich Greenland have completely overshadowed the meeting in Davos, where he landed by helicopter, stepping onto a red carpet laid in the snow.

He arrived some two hours behind schedule, after an electrical issue earlier forced Air Force One to turn back to Washington and switch planes.

But in a sign of the dissent against Trump, the words "No Kings" were dug into the snow overlooking mountain-fringed Davos overnight, referring to a US protest slogan.

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Europe and Canada had earlier closed ranks against what they view as a threat to the US-led global order from Trump's territorial ambitions over semi-autonomous Greenland.

Canada's Prime Minister Mark Carney warned at Davos on Tuesday of a "rupture" to the US-led system, in a speech which won a standing ovation.

NATO chief Mark Rutte told Davos on Wednesday that "thoughtful diplomacy" was needed, as Trump's claims over Greenland provoke an existential crisis for the group.

"There are these tensions at the moment, there's no doubt," said Rutte, who has been dubbed the "Trump whisperer" for his ability to deal with the US president.

Rutte also pushed back against Trump after he said he doubted NATO would come to the aid of the US if asked. "I tell him, yes they will," Rutte said.

Trump however repeated his doubts on NATO during his speech, saying Washington was treated "so unfairly."

France, meanwhile, called Wednesday for a NATO exercise in Greenland "and is ready to contribute to it," French President Emmanuel Macron's office said.

Macron had warned at Davos on Tuesday against US attempts to "subordinate Europe," and blasted Trump's "unacceptable" threats to impose tariffs on allies over Greenland.

Europe has threatened countermeasures after Trump turned up the pressure by threatening levies of up to 25% on eight European countries for backing Denmark.

Greenland's prime minister said Tuesday that the island's population of 57,000 must be prepared for military force.

On Thursday, meanwhile, Trump is set to formally announce the first charter of his so-called "Board of Peace," a body for resolving international conflicts with a $1 billion price tag for permanent membership.