U.S. Vice-President JD Vance says Greenland is critical for the defense of the US and the world against possible Russian or Chinese missile attacks - and that Europe and Denmark have not done a good job in securing the area.

Vance told Fox News that they had not only under-invested in Greenland's defenses, but also failed to engage with President Donald Trump's argument over the issue.

U.S. officials are actively discussing a potential offer to buy Greenland - a semi-autonomous Danish territory - the White House said on Wednesday, a day after suggesting military action to annex it to the U.S. was also an option.

Denmark, a fellow NATO member, has warned this would spell the end of the alliance.

Both Greenland and Denmark have repeatedly stressed the island is not for sale.

Despite being the most sparsely populated territory, Greenland's location between North America and the Arctic makes it well placed for early warning systems in the event of missile attacks, and for monitoring vessels in the region.

Pituffik Space Base, formerly known as Thule Air Base, has been operated by the U.S. since World War II.

In recent years, there has also been increased interest in Greenland's natural resources - including rare earth minerals, uranium and iron - which are becoming easier to access as its ice melts due to climate change. Scientists think it could also have significant oil and gas reserves.

People do not realise that the entire missile defense infrastructure is partially dependent on Greenland, Vance said in his interview on Wednesday.

Concerns over the future of the territory resurfaced after Trump's use of military force against Venezuela on Saturday to seize its President Nicolás Maduro.

A day earlier, European leaders issued a joint statement rallying behind Denmark, asserting that Greenland belongs to its people, and only Denmark and Greenland can decide on matters concerning their relations. They highlighted the significance of collaboration among NATO allies to ensure Arctic security.