Danish Prime Minister Mette Frederiksen warned on Monday that the long-standing NATO alliance would end if President Trump ordered an attack on Greenland, a semiautonomous territory of the Kingdom of Denmark.

In an interview on the Danish broadcaster TV2, Frederiksen responded to Trump’s renewed threats to take over the resource-rich island.

“I believe one should take the American president seriously when he says that he wants Greenland,” Frederiksen said in the interview, according to a translation from Bloomberg.

“But I will also make it clear that if the U.S. chooses to attack another NATO country militarily, then everything stops, including NATO and thus the security that has been established since the end of the Second World War,” she continued.

Trump has frequently toyed with the possibility of taking over Greenland, noting its potential usefulness for U.S. security interests. But those threats took on greater weight this weekend, after the Trump administration carried out a stunning military operation, capturing Venezuelan President Nicolás Maduro and his wife and bringing them to New York to face charges.

Amid questions about which country could face a similar fate, Trump told reporters Sunday about Greenland: “It’s so strategic right now. Greenland is covered with Russian and Chinese ships all over the place.”

“We need Greenland from the standpoint of national security, and Denmark is not going to be able to do it,” the president said.

Frederiksen issued an earlier statement fiercely defending Greenland against Trump’s threats, saying, “It makes absolutely no sense to speak of any necessity for the United States to take over Greenland. The United States has no legal basis to annex one of the three countries of the Kingdom of Denmark.”

Frederiksen noted Greenland, by virtue of being part of Denmark, is part of the NATO alliance and pointed to Denmark’s agreement granting the U.S. broad access to Greenland.

“I therefore strongly urge the United States to cease its threats against a historically close ally, and against another country and another people who have stated very clearly that they are not for sale,” Frederiksen said.