The White House on Tuesday said President Donald Trump and his advisers are actively considering options to acquire Greenland, including the possible use of military force, intensifying tensions with Denmark and other U.S. allies.

White House press secretary Karoline Leavitt said Trump views Greenland as a strategic priority for U.S. national security, particularly in the Arctic region.

“President Trump has made it well known that acquiring Greenland is a national security priority of the United States, and it’s vital to deter our adversaries in the Arctic region,” Leavitt said in a statement. “The President and his team are discussing a range of options to pursue this important foreign policy goal, and of course, utilizing the U.S. Military is always an option at the Commander in Chief’s disposal.”

European leaders have reacted. Danish Prime Minister Mette Frederiksen warned that a U.S. military move against Greenland would have grave consequences for NATO.

“If the United States chooses to attack another NATO country militarily, then everything stops,” Frederiksen said. “That is, including our NATO and thus the security that has been provided since the end of the Second World War.”

She added, “We will not accept a situation where we and Greenland are threatened in this way.”

In a joint statement, leaders of France, Germany, Italy, Poland, Spain, the United Kingdom and Denmark reaffirmed their commitment to “sovereignty, territorial integrity and the inviolability of borders,” stressing that “Greenland belongs to its people” and that decisions concerning the territory rest with Denmark and Greenland alone.

Greenland’s Prime Minister, Jens-Frederik Nielsen, welcomed the show of solidarity and called for a “respectful dialogue” with Washington. Greenland’s Foreign Affairs Minister, Vivian Motzfeldt, said Danish and Greenlandic officials have requested a meeting with U.S. Secretary of State Marco Rubio to address the claims.

Trump has repeatedly argued that Greenland is essential for U.S. defence and access to Arctic trade routes and mineral resources, saying on Sunday, “We need Greenland from a national security situation.”