Donald Trump on Friday warned that he could slap tariffs on countries that do not support his Greenland takeover plans, as US Congress members visited Copenhagen to give their backing for Denmark and its autonomous Arctic island.

The bipartisan delegation, on a two-day trip to the Danish capital, said the US president's long-held territorial ambitions – strongly rejected by Denmark – were not shared by the American people.

Democratic Senator Chris Coons said the purpose of the Congress members' visit was to "listen respectfully to our friends, our trusted allies and partners here in Denmark and from Greenland".

The lawmakers were then to return to the United States "and share those perspectives so that we can lower the temperature and have a more constructive dialogue about the best path forward", he said.

Europeans have also been showing their backing for Greenland, in a military reconnaissance mission that a Danish general said Washington was invited to and which was linked to what Russia does after the war in Ukraine.

"When the war in Ukraine is over, hopefully with a good result for Ukraine, it is our expectation that Russia will move the resources they have been using in Ukraine on other theatres,... including in the Arctic," Major General Soren Andersen told AFP.

The White House has said Trump's aim to take over Greenland would not be affected by the European military presence.

Trump, again insisting the United States needed mineral-rich Greenland for its "national security", warned that he "may put a tariff" on countries that oppose that stance. (AFP)