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.

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

"I mean, NATO would have an obligation to defend Greenland. And so query whether we would be at war with Europe, with England, with France," Sen. Chris Murphy said.

Countries in Europe reportedly are working on a plan if the Trump administration makes a move on acquiring Greenland, which is a a Danish territory.

US Vice-President JD Vance says Greenland is "critical" for the defence of the US and the world against possible Russian or Chinese missile attacks - and that Europe and Denmark have "not done a good job" in securing the area.

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.