President Trump has been urging aides to explore a way to buy Greenland from Denmark, according to three people familiar with the discussions.

His interest in Greenland began last year. At a meeting that spring in the Oval Office, he joked about buying Greenland for its resources, according to a person who was in attendance.

In the year since, the President has repeatedly returned to the topic, asking aides if they can pursue a purchase of Greenland, a semiautonomous territory that Trump has been taken with in part because of its natural resources, like coal and uranium.

Greenland on Friday dismissed the notion that it might be up for sale. “We are open for business, but we’re not for sale,” Greenland’s foreign minister Ane Lone Bagger told Reuters.

Privately, Trump’s advisers are highly sceptical that such a move could ever happen. But instead of telling him they do not think it is possible, the advisers have agreed to investigate the matter, according to the people briefed on the discussions.

His interest in buying Greenland was first reported Thursday by The Wall Street Journal. Greenland, a self-governing country, is part of the kingdom of Denmark, where Trump is scheduled to visit in September to meet with the Prime Minister, Mette Frederiksen, Queen Margrethe II and the leaders of Greenland and the Faroe Islands.

The White House declined to comment.

Trump is a former real estate developer who tends to view his presidency through the lens of acquisitions. But the territory appeals to him partly because of its national security value given its location, according to two of the people briefed on the discussions.

Danish politicians on Friday poured scorn on the idea.

“It has to be an April Fool’s joke. Totally out of season,” former Prime Minister Lars Lokke Rasmussen said on Twitter.

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

“The thought of Denmark selling 50,000 citizens to the US is completely ridiculous,” he said. Greenland, a self-ruling part of Denmark located between the North Atlantic and Arctic oceans, is dependant on Danish economic support. It handles its own domestic

affairs while Copenhagen looks after defence and foreign policy.

“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, told Reuters. “My immediate thought is ‘No, thank you’,” she said. Prime Minister Mette Frederiksen was not available for comment.