Denmark Prime Minister Mette Frederiksen said "you cannot spy against an ally" after reports that the U.S. was gathering intelligence on Greenland.

Denmark and Greenland insist that the mineral-rich island is not for sale, while President Trump has not ruled out taking it by military force even though Denmark is a NATO ally.

Denmark summoned the top U.S. diplomat in the country for an explanation following a Wall Street Journal report about the United States stepping up its intelligence gathering on Greenland, a semi-autonomous Danish territory coveted by President Donald Trump.