President Trump and his administration are exploring “a range of options” to acquire Greenland, including the potential use of the U.S. military, though Secretary of State Marco Rubio told lawmakers that the preferred approach is to purchase the island from Denmark.

The president has argued that Greenland is strategically vital for U.S. national security, citing Russian and Chinese activity in the Arctic, while Denmark and other European NATO leaders have firmly rejected any U.S. claims.

White House aides, including Stephen Miller, have refused to rule out military action, prompting concern among lawmakers who warn that NATO obligations could require member countries to defend Greenland.

Critics, including several U.S. senators, have introduced or called for resolutions to prevent any military move, describing Trump’s comments as increasingly serious and alarming, CNBC has reported.