A European Commissioner has said that it would be the end of NATO if the United States took Greenland by force.
Commissioner for Defence and Space Andrius Kubilius said that European Union member states would be obliged to come to Denmark's assistance if faced with military aggression.
"I agree with the Danish Prime Minister (Mette Frederiksen) that it will be the end of NATO, but also among people it will be very, very negative," he said as he attended a security conference in Sweden.
Earlier, Ms Frederiksen said that Denmark faced a "decisive moment" in its diplomatic battle with the US over Greenland, after President Donald Trump again suggested using force to seize the Arctic territory.
"There is conflict over Greenland," she said, ahead of meetings in Washington.
"This is a decisive moment" with stakes that go beyond the immediate issue of the territory's future, she added in a debate with other Danish political leaders.
Ms Frederiksen posted on Facebook that "we are ready to defend our values - wherever it is necessary - also in the Arctic. We believe in international law and in people's right to self-determination".
NATO Secretary General Mark Rutte said that member states are working on the next steps to collectively keep the Arctic safe.
The alliance has previously discussed how to work together on the issue, Mr Rutte told a news conference in Croatia.
"Currently, we are discussing the next step to that, how to make sure that we give practical follow up on those discussions.
"We are working on the next steps to make sure that indeed, we collectively protect what is at stake here," Mr Rutte said.
Watch: Trump says US will take Greenland 'one way or the other'
Germany and Sweden backed Denmark against President Trump's latest claims to the self-governing Danish territory.
Before meeting US counterpart Marco Rubio, German Foreign Minister Johann Wadehpul held talks in Iceland to address the "strategic challenges of the Far North," according to a foreign ministry statement.
"Security in the Arctic is becoming more and more important" and "is part of our common interest in NATO," he said at a joint news conference with Icelandic Foreign Minister Thorgerdur Katrin Gunnarsdottir.
"If the American president is looking at what threats might come from Russian or Chinese ships or submarines in the region, we can of course find answers to that together."
However, "the future of Greenland must be decided by the people of Greenland" and Denmark, he said.
Asked about a possible strengthening of NATO's commitment in the Arctic, Mr Wadephul said Germany was "ready to assume greater responsibilities".
Earlier, German Finance Minister Lars Klingbeil said: "We are strengthening security in the Arctic together, as NATO allies, and not against one another."
He was speaking ahead of an international meeting on critical raw materials in Washington.
Swedish Prime Minister Ulf Kristersson condemned the United States for using "threatening rhetoric" after he repeated that the US was "going to do something on Greenland, whether they like it or not".
"Sweden, the Nordic countries, the Baltic states, and several major European countries stand together with our Danish friends," he told a defence conference in Sweden where the US general in charge of NATO took part.
Mr Kristersson said that a US takeover of mineral-rich Greenland would be "a violation of international law and risks encouraging other countries to act in exactly the same way".
Watch: RTÉ's Deputy Foreign Editor Edmund Heaphy reports from the Greenland capital, Nuuk
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European nations have scrambled to coordinate a response after the White House said this week that Mr Trump wanted to buy Greenland and refused to rule out military action.
On Tuesday, leaders of seven European countries, including France, the UK, Germany and Italy, signed a letter saying it is "only" for Denmark and Greenland to decide the territory's future.
The US president has said controlling the island is crucial for US national security because of the rising Russian and Chinese military activity in the Arctic.
NATO Supreme Allied Commander General Alexus Grynkewich told the Swedish conference that alliance members were discussing Greenland's status.
While there was "no immediate threat" to NATO territory, the Arctic's strategic importance was fast growing, the US general added.
Mr Grynkewich said he would not comment on "the political dimensions of recent rhetoric" but talks on Greenland were being held at the North Atlantic Council.
"Those dialogues continue in Brussels. They have been healthy dialogues from what I've heard," the general said.
A Danish colony until 1953, Greenland gained home rule 26 years later and is contemplating eventually loosening its ties with Denmark. Polls indicate that Greenland's population strongly opposes a US takeover.
"I don't think there's an immediate threat to NATO territory right now," Mr Grynkewich told the conference.
However, he said Russian and Chinese vessels had been seen patrolling together on Russia's northern coast and near Alaska and Canada, working together to get greater access to the Arctic as ice recedes due to global warming.
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