United States President Donald Trump has announced that large-scale exploration for oil, gas and minerals is set to begin in Greenland, describing the Arctic island's reserves as among the largest in the world.

In a post on his Truth Social platform on Wednesday, Trump said Greenland holds some of the world's largest reserves of "easily" extractable oil.

"Massive Oil, Gas, and Mineral Exploration is getting ready to begin in our new and beautiful Greenland!" Trump wrote.

"The amount of easily available Oil is far greater than that of almost … anywhere in the World," he added, citing experts. He did not provide details of potential US involvement in the exploration.

Trump also praised the security advantages that both the United States and Europe have gained from a deal signed with Denmark and Greenland last month, describing them as "second to none."

The agreement, reached last month, expands the role of the US military on the island. It paves the way for two new military bases and Trump's planned "Golden Dome" missile defence system, while reaffirming Danish sovereignty and Greenlanders' right to self-determination.

The deal was aimed at ending an almost two-year-long crisis stemming from Trump's repeated refusal to rule out the United States seizing Greenland by force.

In his Wednesday post, Trump argued that Greenland's geographic position makes it essential to American defence.

"Any future attack on the United States will have to come across Greenland, where our defence will routinely knock down drones, missiles, or any other form of weapon not yet even conceived," he said.

"It gives us time, location, and sightline, like nowhere else in the world!"

There was no immediate comment from the leaders of Greenland or Denmark following the president's post.