Trump’s threat to annex an autonomous part of Denmark has plunged NATO into an unprecedented situation: An alliance based on collective defense now faces the prospect that one member might attack another.

Allies including France and Germany are working closely on a plan on how to respond should the United States act on its threat to take over Greenland.

Trump administration officials are set to meet with Danish officials about Greenland on Wednesday, diplomatic sources tell CBS News.

Trump allies have cited the island's continued role in missile defense infrastructure, accusing Denmark of leaving it vulnerable to potential Russian and Chinese influence.

The White House has kept all options for acquiring Greenland on the table, refusing to rule out military action.

Europe, led by the UK and Germany is exploring a stronger NATO presence in Greenland to reinforce Arctic security following US President Donald Trump’s call for American ownership and reported invasion planning.

U.S. Secretary of State Marco Rubio on Wednesday said that he would meet with Denmark next week, as U.S. President Donald Trump has repeated in recent days that he wants to gain control of Greenland.

President Donald Trump’s renewed interest in the United States acquiring Greenland has stirred new concerns in Europe and NATO.