US President Donald Trump demanded immediate talks on acquiring Greenland in a hardline speech at Davos Wednesday, but said that while the United States 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."

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."

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

"I love Europe and I want to see Europe go good, but it's not heading in the right direction... We want strong allies, not seriously weakened ones."

His arrival in the Swiss Alps community of Davos was delayed after a minor electrical issue aboard Air Force One had forced a return to Washington to switch aircraft.

Trump is expected to have around five bilateral meetings with foreign leaders, though further details weren't provided. (AP, AFP)