Washington DC - A Republican congressman from Florida introduced a bill Monday to annex Greenland and make it the 51st US state as President Donald Trump threatens to seize the autonomous Danish territory.

Trump has said the US will take Greenland "one way or the other," floating the idea of buying the territory or taking it by force.

Representative Randy Fine said his new legislation – dubbed the Greenland Annexation and Statehood Act – would authorize Trump "to take whatever steps necessary to annex or acquire Greenland."

The bill would also require the Trump administration to send a report to Congress detailing changes to federal law that would allow the Arctic island to become a US state.

"Greenland is not a distant outpost we can afford to ignore – it is a vital national security asset," Fine said in a statement.

Trump has insisted that Greenland – rich in rare earth mineral resources – needs to be brought under US control, arguing the territory is crucial for national security.

The president has said Russia or China will take Greenland if the US does not. Neither Moscow nor Beijing has threatened or laid any claim to the island.

The vast majority of Greenland's population and political parties have said they do not want to be under US control and insist Greenlanders must decide their own future

Democrats and Republicans alike have pushed back on his rhetoric, and Danish Prime Minister Mette Frederiksen has warned that an armed attack on Greenland from the US would spell the end of NATO, the 32-member military alliance.

NATO and Greenland's government on Monday said they intended to work on strengthening the defense of the territory.