Mette Frederiksen, the prime minister of Denmark, said on Wednesday that the country would defend “every inch” of its territory and reiterated that Greenland was not for sale as President Trump again said that the United States should have control over the island.

As she has done before, Ms. Frederiksen shut down the idea, telling reporters at the NATO summit in Turkey that Greenland was not for sale. When asked how she would respond if the island were attacked, she said, “We will defend the kingdom of Denmark,” adding, “We are ready to defend every inch of NATO, including our own territory.”

Mr. Trump on Wednesday expressed frustration with NATO over the issue. “Greenland is a big problem for us,” he said. “We need it for protection of the world, not just the United States, and it’s very important. It doesn’t help Denmark.”

In January, Mr. Trump suggested that he could use military force to take control of the island. In response, Denmark developed plans to blow up airfields in Greenland in the event of an American invasion, and Ms. Frederiksen openly wondered how long the United States would remain an ally.

In recent months, Mr. Trump has backed off his most serious threats, with Washington turning to negotiations to press for more influence — including pushing Denmark for access to three more military bases there, demanding effective veto power over any major investment deals and opening a bigger consulate in Nuuk, the island’s capital.

Mr. Trump’s proposals seem to have drawn Greenland closer to Denmark as they try to present a united front against the American advances.

On Wednesday, Ms. Frederiksen repeated Denmark’s opposition.

“We are a sovereign state and we need everyone to respect our territorial integrity and our sovereignty,” she said.

“Article 5 is our insurance,” she added, referring to NATO’s collective defense agreement.