U.S. Sen. Thom Tillis, a North Carolina Republican, forcefully rebutted comments from one of the top officials in the Trump administration about a potential American takeover of Greenland, an idea that has been floated since Trump returned to the Oval Office.

Tillis, who is not running for another term, called comments made by Deputy White House Chief of Staff Stephen Miller “insane” and “amateur hour.” In an interview, Miller said “the formal position of the US government” is that Greenland should be part of the United States.

Greenland is part of Denmark, a European ally and fellow member of the North Atlantic Treaty Organization (NATO). Tillis is a staunch defender of the NATO alliance.

“By turning around and making insane comments about how it is our right to have territory owned by the, the kingdom of Denmark, folks, amateur hour is over,” Tillis said from the Senate floor Tuesday. “You don't speak on behalf of this U.S. senator or the Congress.”