"You know, Mark, we need that for international security, not just security - international - we have a lot of our favourite players cruising around the coast, and we have to be careful," Trump told Rutte on Thursday, as they sat side-by-side in the White House Oval Office for talks. "We'll be talking to you."
Asked directly about the prospect of annexation, Trump said: "I think that will happen."
Trump has made US annexation of Greenland a major talking point since he took office on January 20.
His comments on Thursday suggested he might want NATO involved in his attempt to take over the island, a semi-autonomous Danish territory.
The comments drew a swift rejection from the outgoing prime minister of Greenland.
"The US president has once again aired the thought of annexing us," Mute Egede said in a Facebook post. "Enough is enough."
NATO and Denmark's embassy in Washington did not immediately respond to a request for comment.
Opinion polls suggest most Greenlanders oppose joining the US, although a majority favour eventual independence from Denmark.
Greenland's pro-business opposition Demokraatit party, which wants a slow approach to independence from Denmark, won Tuesday's parliamentary election. On Thursday, Trump said that election was good for the United States.
Even before starting his second term as president, Trump said he hoped to make Greenland a part of the United States, even though NATO ally Denmark says it is not for sale.
Greenland's strategic location and rich mineral resources could benefit the US It lies along the shortest route from Europe to North America, vital for the US ballistic missile warning system.
Trump has managed to annoy Canadians with his proposal the country become the 51st US state. He also demands the US exert more influence over the Panama canal.
Rutte told Trump he would leave the question of Greenland's future to others and that, "I don't want to drag NATO" into the debate. He said it should be a topic for countries in the "high north" because the Chinese and Russia are using water routes in the area.
Trump persisted, saying Denmark was refusing to discuss the topic and he might send more US troops to bolster the American bases on Greenland.
"We've been dealing with Denmark, we've been dealing with Greenland, and we have to do it. We really need it for national security. I think that's why NATO might have to get involved in a way, because we really need Greenland for national security. It's very important," Trump said.
Trump also sought to undermine Denmark's claim to the island.
"You know, Denmark's very far away, and really has nothing to do. What happens, a boat landed there 200 years ago or something? And they say they have rights to it. I don't know if that's true. I don't think it is," he said.
