Denmark announced on Friday that it took seriously its statement that Greenland was not for sale after Secretary of State Marco Rubio stated President Donald Trump’s interest in purchasing the island “wasn’t a joke.”

Trump has said that he plans to make the autonomous territory in Denmark part of the United States. He hasn’t ruled out the use of military or economic force to convince Denmark to give it up.

Rubio said on Sirius XM’s The Megyn Kelly Show that acquiring Greenland is in the national interest of the United States. He said that Trump hadn’t ruled out the possibility of military coercion to keep leverage on the table.

“This is not a joke,” Rubio said. “This is not about acquiring land to acquire land. This is in our national interest and it needs to be solved.”

Lars Lokke Rasmussen, the Danish Foreign Minister, responded to Rubio’s remarks by saying: “I’d be more surprised if it had been a joke.”

“We seriously mean – and this is also true in Greenland – that Greenland is not for sale.”

Around 57,000 people live on the island and are responsible for their affairs. Denmark is responsible for Greenland’s defense and safety, but only Greenlanders have the right to decide their future. Under a treaty, the United States maintains an airbase in Greenland.

Opinion polls indicate that most Greenlanders are in favor of a more relaxed relationship with Denmark, but they also do not want the territory to become part of the United States.

Greenland’s Prime Minister Mute Ede, who is intensifying his push for independence, has said repeatedly that the island was not for sale, and that its people had to decide their fate.

Rubio said in an interview that the Arctic will become a critical shipping lane and the United States must be able to defend it. He suggested that the U.S.’s rival China might seek to expand its presence.

Rubio responded: “Obviously, that’s President Obama’s priority. He has stated that. We are not yet in a position to discuss how we will proceed tactically.” You can be assured that in four years, the U.S. will have a stronger interest in Greenland.

Rasmussen stated that Rubio’s interview outlines U.S. interests which are the same as those of the Danish Kingdom.

He said, “If we have a substantive conversation about this issue, we’ll also find a resolution.”

Rasmussen, referring to Rubio’s remarks, said: “It can be summarized in the ambition that everything would fall under control if only the United States owned the entire world. This is not possible, so we must find a way to accomplish our tasks together.