Danish politicians poured scorn on the idea of selling Greenland to the U.S. after reports President Donald Trump had privately discussed the idea of buying the world's biggest island with his advisers.

Trump is due to visit Copenhagen in September. The Arctic will be on the agenda during meetings with the prime ministers of Denmark and Greenland, an autonomous Danish territory.

The notion of purchasing the territory has been laughed off by some advisers as a joke, but was taken more seriously by others in the White House, two sources familiar with the situation told Reuters on Thursday.

Talk of a Greenland purchase was first reported by the Wall Street Journal.

'It has to be an April Fool's joke'

"If he is truly contemplating this, then this is final proof, that he has gone mad," Soren Espersen, the foreign affairs spokesperson for the Danish People's Party, told broadcaster DR.

"The thought of Denmark selling 50,000 citizens to the United States is completely ridiculous," he said.

"It has to be an April Fool's joke. Totally out of season," former prime minister Lars Lokke Rasmussen said on Twitter.

It must be an April Fool’s Day joke ... but totally out of sesson! <a href="https://t.co/ev5DDVZc5f">https://t.co/ev5DDVZc5f</a> —@larsloekke

Greenland, a self-ruling part of Denmark located between the North Atlantic and Arctic oceans, is dependent on Danish economic support.

"I am sure a majority in Greenland believes it is better to have a relation to Denmark than the United States, in the long term," Aaja Chemnitz Larsen, Danish MP from Greenland's second largest party, Inuit Ataqatigiit (IA), told Reuters.

"My immediate thought is 'No, thank you.'"

Greenland's strategic proximity to the Arctic and its natural resources have also caught the attention of global powers like China and Russia. (The Associated Press)

Officials said Prime Minister Mette Frederiksen will respond later on Friday. The U.S. Embassy in Copenhagen was not immediately available for comment.

"We are open for business, but we're not for sale," Greenland's foreign affairs minister, Ane Lone Bagger, told Reuters.

"Oh dear lord. As someone who loves Greenland, has been there nine times to every corner and loves the people, this is a complete and total catastrophe," Rufus Gifford, the former U.S. ambassador to Denmark, said on Twitter.

U.S. has airbase on the island

In May, U.S. Secretary of State Mike Pompeo said Russia was behaving aggressively in the Arctic and China's actions there had to be watched closely as well.

A defence treaty between Denmark and the United States dating back to 1951 gives the U.S. military rights over the Thule Air Base in northern Greenland.

Greenland is part of Denmark with self-government over domestic affairs, while Copenhagen handles defence and foreign policy.

The world's largest island, Greenland is close to the Arctic, a region with strategic implications for several global powers. (Google Earth)

There has been no indication a Greenland purchase will be on the agenda for Trump's talks with Danish officials.

Martin Lidegaard, senior lawmaker of the Danish Social Liberal Party and a former foreign affairs minister, called the idea "a grotesque proposal" with no basis in reality.

"We are talking about real people and you can't just sell Greenland like an old colonial power," he told Reuters.

"But what we can take seriously is that the U.S. stakes and interest in the Arctic is significantly on the rise and they want a much bigger influence."

In 1917, Denmark sold off the then Danish West Indies islands for $25 million to the United States, which renamed them the United States Virgin Islands.