Denmark’s foreign minister said there were “fundamental differences” between Denmark and the United States over Greenland following White House talks.

Ras Roch Rasmussen said Wednesday’s meeting with Vice President Vance and Secretary of State Marco Rubio was “candid but constructive.” But he added that US President Donald Trump’s insistence on “conquering” Greenland was “completely unacceptable”.

“We made it very, very clear that this is not in (Denmark’s) interests,” he said.

Trump later reiterated his interest in acquiring the resource-rich island, a stance that has unsettled European allies and heightened tensions with Nato.

The hour-long meeting between the United States, Denmark and Greenland failed to produce a major breakthrough, although the parties agreed to establish a high-level working group to discuss the future of the self-governing Danish territory.

Rasmussen said the United States cannot cross a “red line” and that the working group will meet in the coming weeks to try to find a compromise. He added that Denmark and Greenland were open to the possibility of the United States opening more military bases on the islands.

Although Greenland is the most sparsely populated region, its location between North America and the Arctic makes it ideal for setting up an early warning system in the event of a missile attack and monitoring ships in the area.

Trump has repeatedly stated that Greenland is vital to U.S. national security.

The United States already has more than 100 military personnel permanently stationed at Pitufik Base on the northwest tip of Greenland, which has been operated by the United States since World War II.

Under an existing agreement with Denmark, the United States has the right to send as many troops as possible to Greenland.

After the meeting, Denmark vowed to send armed forces to the island.

Greenland’s Foreign Minister Vivian Motzfeldt said the territory was open to greater cooperation with the United States but also opposed a takeover.

“We’ve shown where our limits are,” Motzfeldt said.

Vance and Rubio had no immediate comment after the meeting. “We need Greenland for our national security,” Trump told reporters in the Oval Office.

“The problem is, if Russia or China wants to occupy Greenland, Denmark can’t do anything, but we can do everything,” he said, adding that he did not believe the United States could rely on Denmark to defend the island.

As tensions rise in Washington, European allies have moved quickly to show support for Greenland.

Sweden pledged on Wednesday to send armed forces to Greenland at Denmark’s request. France’s foreign minister said the country plans to open a consulate on the island next month.

In a statement provided to the BBC, Germany said it would send a “reconnaissance team” to Greenland to “explore general conditions for possible military assistance to support Denmark in ensuring security in the region”.

Denmark said its military expansion in Greenland would be carried out “in close cooperation with allies”.

“Geopolitical tensions have spread to the Arctic,” it said in a statement.

The White House talks are just the latest round of diplomatic discussions as Trump becomes increasingly interested in taking control of Greenland in some way.

It is unclear whether the president is considering using force to seize the island. When asked on Wednesday, he refused to rule it out.

Other options reportedly being considered include buying the territory – although neither the Danes nor the Greenlanders have indicated they want to sell it.

Since taking office, Vance has been critical of Denmark’s management of Greenland. The vice president’s visit, which came shortly after taking office, was intended to signal the government’s interest in acquiring the territory.

Trump has argued that controlling the territory is vital to his planned “Golden Dome” missile defense system.

“This is critical to the Golden Dome we are building,” Trump said in a social media post early Wednesday. “NATO should lead the way for us to achieve this goal.”

Opinion polls show that a majority of Greenland residents oppose accepting U.S. control.

Most Americans also oppose U.S. control of Greenland. A Reuters/Ipsos poll released on Wednesday found that only 17% of Americans support the U.S. occupation of Greenland, while 47% said they oppose Trump’s push to occupy the island.

Trump’s focus on Greenland follows U.S. attacks against Islamic State in Venezuela and Syria in recent weeks. Trump also threatened military action to stop Iran’s deadly crackdown on the protest movement sweeping the country.