Denmark summoned the top U.S. diplomat in Copenhagen on Wednesday after Danish media reported that Americans with ties to President Donald Trump had carried out covert influence operations in Greenland.

Danish broadcaster DR reported that at least three U.S. citizens linked to the U.S. government were involved in activities that, reportedly, authorities fear could be used covertly to support Trump’s desire to make Greenland part of the United States.

Foreign Minister Lars Løkke Rasmussen said the U.S. chargé d’affaires — currently its most senior diplomat in Denmark — had been summoned in response. He called any interference in Danish affairs “unacceptable,” and emphasized that Copenhagen “will of course not accept covert operations on our territory,” in a statement emailed by his ministry, according to the AP.

“It worries me greatly because we do not spy on friends,” Rasmussen also said in response to a report in The Wall Street Journal.

According to DR, one of the men compiled lists of Greenlanders supportive of, or critical toward, U.S. influence, while others maintained political and business contacts on the island. It was unclear whether they acted independently or under direction from U.S. officials.

The move comes amid ongoing tensions over Greenland, a mineral-rich, self-governing Danish territory. Earlier this year, Trump told CNN that Washington would “100 percent” gain control of Greenland, even repeatedly threatening to use military force.

Greenland is strategically important for U.S. military and Arctic security interests. Contacted by DR, Denmark’s security and intelligence service, known as PET, said the territory “is the target of influence campaigns of various kinds” and had strengthened monitoring in cooperation with Greenlandic authorities.