President Donald Trump told NATO Secretary-General Mark Rutte on Thursday that U.S. control of Greenland is necessary to enhance international security, escalating his campaign to annex the strategic Arctic island.

Both Canada and Greenland, which is included in NATO as the territory of founding member Denmark, were raised during an informal press conference in the Oval Office. In the meeting, Trump also applied new pressure on the trans-Atlantic alliance to spend more on its collective defense.

The massive Arctic island is mostly empty, but there are still plenty of reasons why the president would want to bring it under American control.