Trump's threats over the vast Arctic island -- an autonomous territory of NATO member Denmark -- plunged relations between Europe and its key ally Washington to a historic low.

While the immediate danger appeared to recede after Trump struck a "framework" agreement with NATO chief Mark Rutte, the bloc maintained a meeting called to address the crisis -- and how to handle the US leader going forward.

It was still unclear what exactly prompted Trump's U-turn, but French President Emmanuel Macron insisted a push to unleash the European Union's trade arsenal against the United States played a part -- and said Europe remains "extremely vigilant".

Addressing a news conference after the summit, European Council chief Antonio Costa vowed the bloc "will continue to stand up for its interests" and defend itself "against any form of coercion."

"It has the power and the tools to do so, and will do so if and when necessary."

- Details scarce -

Trump backed down Wednesday night both on threatening to seize Greenland by force and on imposing tariffs against European allies, touting a "framework" agreement that would give Washington "everything we wanted".

Costa described the about-turn on tariffs as "positive" and called for the trade deal agreed with Washington last summer -- which senior European lawmakers had vowed to block in response to US threats -- to be fully rolled out.

"The goal remains the effective stabilisation of the trade relations between the European Union and the US," Costa said, summing up what EU leaders concluded during the evening talks.

NATO's Rutte appeared to have pulled off a diplomatic coup by talking Trump down from his demands -- but swirling questions remained about their purported deal, and what might follow.

Details remain scant, with Greenland's Prime Minister Jens-Frederik Nielsen saying he did not know what had been agreed.

But a source familiar with the talks told AFP the United States and Denmark would renegotiate a 1951 defence pact on Greenland.

In Brussels, Danish Prime Minister Mette Frederiksen told reporters the matter of Denmark's sovereignty was off the table, but that Copenhagen was open to discussing the pact with the United States.

She also said NATO states backed having a "permanent presence" in the Arctic, including around Greenland.

The Danish leader -- who has maintained close contact with Rutte -- was to meet the NATO chief for a debrief Friday morning in Brussels.

- 'A tweet away' -

Europe has struggled to set red lines as its once-close American ally has turned hostile under Trump -- to the point of threatening its sovereignty.

"You know we're only a tweet away from the next crisis," summed up one EU diplomat.

Greenland is only part of the picture, as Trump wages a broader attack on the EU's laws, politics and values, zig-zags on the Ukraine conflict, or pushes a self-styled "Board of Peace" after a year of tearing down global norms.

EU leaders had "serious doubts about a number of elements" of the latter initiative, including its scope, governance and compatibility with the UN Charter, Costa said.

Polish Prime Minister Donald Tusk said the importance of preserving transatlantic ties was "absolutely obvious" for all Europeans.

"But what we need today in our politics is trust and respect among all partners, not domination, and for sure, not coercion," he said.

The continent is ramping up defence spending to break its security reliance on the United States -- but for now, it still needs US help to end the Ukraine war, and deter the looming Russian threat to its east.

With a transatlantic clash over Greenland at least temporarily defused, the EU's top diplomat Kaja Kallas said it was "important to focus back on where the problems really lie -- and this is the war in Ukraine."

But as leaders met in Brussels, Ukrainian President Volodymyr Zelensky aimed a brutal warning at the bloc, saying Trump "will not listen to this kind of Europe" unless it steps up to become "a truly global power."