Washington DC - President Donald Trump is serious about seeking to buy Greenland, Secretary of State Marco Rubio said Thursday, after Denmark was rattled by his threats to take over the autonomous Danish territory.

In an interview, Rubio played down the threat of the US using military force against Denmark, a NATO ally, but said of Trump's remarks on Greenland, "This is not a joke."

"President Trump's put out there what he intends to do, which is to purchase it," Rubio told SiriusXM Radio.

"This is not about acquiring land for the purpose of acquiring land. This is in our national interest, and it needs to be solved," Rubio said.

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Referring to NATO guarantees to Denmark, Rubio said: "We have a defense agreement with them to protect Greenland if it becomes under assault."

"If we're already on the hook for having to do that, then we might as well have more control over what happens there," he said.

Danish Prime Minister Mette Frederiksen has rallied European allies over the threats from Trump, with whom she was said to have had a tense telephone conversation.

Rubio said that while he was not on the phone call with Frederiksen, Trump "just speaks bluntly and frankly with people."

"And ultimately, I think diplomacy in many cases works better when you're straightforward as opposed to using platitudes and language that translates to nothing," Rubio said.

Rubio voiced concern that rival China, looking for Arctic access, would gain ground in Greenland through state-run companies.

"It is completely realistic to believe that the Chinese will eventually, maybe even in the short term, try to do in Greenland what they have done at the Panama Canal and in other places," Rubio said.