'Keep dreaming': NATO chief says Europe can't defend itself without US

US President Donald Trump roiled the transatlantic alliance by threatening to seize the autonomous Danish territory -- before backing off after talks with Rutte last week.

The diplomatic crisis sparked gave fresh momentum to those advocating for Europe to take a tougher line against Trump and break its military reliance on Washington.

"If anyone thinks here again, that the European Union, or Europe as a whole, can defend itself without the US -- keep on dreaming. You can't," Rutte told lawmakers at the European Parliament.

He said that EU countries would have to double defence spending from the five percent NATO target agreed last year to 10 percent and spend "billions and billions" on building nuclear arms.

"You would lose the ultimate guarantor of our freedom, which is the US nuclear umbrella," Rutte said. "So hey, good luck."

The former Dutch prime minister insisted that US commitment to NATO's Article Five mutual defence clause remained "total", but that the United States expected European countries to keep spending more on their militaries.

"They need a secure Euro-Atlantic, and they also need a secure Europe. So the US has every interest in NATO," he said.

The NATO head reiterated his repeated praise for Trump for pressuring reluctant European allies to step up defence spending.

He also appeared to knock back a suggestion floated by the EU's defence commissioner Andrius Kubilius earlier this month for a possible European defence force that could replace US troops on the continent.

"It will make things more complicated. I think (Russian President Vladimir) Putin will love it. So think again," Rutte said.

On Greenland, Rutte said he had agreed with Trump that NATO would "take more responsibility for the defence of the Arctic", but it was up to Greenlandic and Danish authorities to negotiate over US presence on the island.

"I have no mandate to negotiate on behalf of Denmark, so I didn't, and I will not," he said.

Rutte reiterated that he had stressed to Trump the cost paid by NATO allies in Afghanistan after the US leader caused outrage by playing down their contribution.

"For every two American soldiers who paid the ultimate price, one soldier of an ally or a partner, a NATO ally or a partner country, did not return home," he said.

"I know that America greatly appreciates all the efforts."