Denmark Prime Minister Mette Frederiksen said Friday that “you cannot spy against an ally” in response to reports that the U.S. was gathering intelligence on Greenland, as U.S. President Donald Trump has continued to suggest purchasing the Arctic island.

“Cooperation about defense and deterrence and security in the northern part of Europe is getting more and more important,” Frederiksen told The Associated Press. “Of course, you cannot spy against an ally.”

Frederiksen made the comments as Denmark and Greenland push back on Trump’s desire to acquire the autonomous Danish territory, stressing that it is not for sale. Trump, however, has not ruled out taking it by military force despite Denmark being a NATO ally.

Officials under the U.S. director of national intelligence, Tulsi Gabbard, directed intelligence agency heads to collect information on Greenland’s independence movement and views about U.S. resource extraction. (Getty Images)

Gabbard’s office released a statement saying she had made three “criminal” referrals to the U.S. Justice Department over intelligence community leaks in response to the report from The Wall Street Journal, which cited two sources familiar with the matter.

“The Wall Street Journal should be ashamed of aiding deep state actors who seek to undermine the President by politicizing and leaking classified information,” Gabbard said. “They are breaking the law and undermining our nation’s security and democracy. Those who leak classified information will be found and held accountable to the fullest extent of the law.”

The Associated Press contributed to this report.