US President-elect Donald Trump is serious about trying to acquire Greenland, both to expand America's sphere of influence in the Western Hemisphere and as a way to cement his legacy, according to three sources familiar with his thinking.

The Republican said on Tuesday he would not rule out using military or economic action to acquire the overseas Danish territory after he takes office on Jan. 20, startling European allies who are concerned about maintaining the inviolability of borders at a time of increased global volatility.

While Trump briefly floated the possibility of acquiring Greenland during his first term, his recent comments underline how his desire is a deep-rooted ambition - not a fleeting fancy.

One person briefed on internal discussions with Trump told Reuters that the former real estate developer is unlikely to use military force to gain control over Greenland.

Still, Trump is serious about using other means - like applying diplomatic or economic pressure on Denmark - to acquire the world's largest island that is not a continent, the person said.

"The real legacy is you have expanded the United States," the person said. "Literally in 70 years we haven't added one piece of real estate to the portfolio. He talks about it a lot."

In 1959, two US territories - Alaska and Hawaii - became the 49th and 50th US states under Republican President Dwight Eisenhower.

Another Trump associate recounted seeing a list of foreign policy priorities put together by top Trump officials following his victory in the Nov. 5 presidential election. One immediately stood out to him: "Buying Greenland."

Trump has portrayed a potential Greenland acquisition as a national security imperative.

Denmark has responded to Trump's comments by saying Greenland is not for sale. Denmark has controlled Greenland for hundreds of years although its 57,000 people now govern their own domestic affairs.

Trump advisers said they expected a shift in US foreign policy resources and attention toward the Americas under a new Trump administration. Trump is suspicious of Chinese and Russian influence in the region, which he sees as essentially America's backyard, two Trump advisers who have been involved in Latin America policy planning said.

The three current and former Trump associates who are familiar with his thinking said Trump is also interested in territorial expansion as a means of creating a lasting legacy that will be remembered for generations.

Trump has publicly floated takeovers of Greenland and the Panama Canal and the absorption of Canada in the United States in recent weeks.

"Every decision President Trump makes is in the best interest of the United States and the American people," Karoline Leavitt, a Trump spokeswoman, said in a statement to Reuters. "That’s why President Trump has called attention to legitimate national security and economic concerns regarding Canada, Greenland, and Panama."