US President Donald Trump said NATO "would have been in the ash heap of History," if it were not for him.
In a post on his Truth Social network, he also claimed no president — or person — did more for the military alliance.
Trump's push for US control over Greenland, a territory of fellow-NATO member Denmark, has prompted warnings that such a move could end the alliance that has underpinned Western security for decades.
Anders Fogh Rasmussen warned the time for "flattering" the US leader was over.
"It's really the future of NATO that is at stake," said Rasmussen, a former leader of both Denmark and NATO.
"The time for flattering is over. It doesn't work. The fact is, Trump only respects force and strength. And unity. That's exactly what Europe should demonstrate right now," he told Reuters news agency at the World Economic Forum in Davos, Switzerland.
NATO only as strong as the US, Trump says
Ahead of a trip to Davos to participate in the World Economic Forum, US President Donald Trump said he would hold several meetings to address his intention to take control of Greenland.
"We have a lot of meetings scheduled on Greenland, and I think it's going to work out," Trump said during a press conference on Tuesday in which he lauded his performance over the first year of his second term.
Asked if taking control of Greenland was worth risking the potential breakup of the NATO military alliance, Trump said he was confident an agreement could be reached.
"Something is going to happen that is going to be good for everybody," he said. "I think we will work something out and that NATO will be very happy and we will be very happy, but we need it for security purposes."
Earlier in the day, Trump had said there was "no going back" on his goal to control Greenland, refusing to rule out taking the Arctic island by force.
Trump also said he had improved NATO by calling on members to spend 5% of their GDP on defense and had made the alliance "so much better, so much stronger."
"NATO is only as good as we are," he said of the United States. "If NATO doesn't have us, NATO is not very strong."
Trump questions NATO's commitment to defend US
President Donald Trump on Tuesday questioned whether NATO would back the United States if it called on the military alliance.
"The big fear I have with NATO is that we spend tremendous amounts of money with and I know we will come their rescue and I just really question whether or not they will come to ours," Trump said at the White House press room while speaking about the accomplishments of his first year back in power.
His assessment comes despite NATO's collective defense agreement, known as Article 5, which states that an attack against one member shall be considered an attack against all. Article 5 has only been triggered once, after the September 11, 2001, terror attacks on the US, when all its allies positively responded.
The remarks also come as Trump is pushing for control of Greenland from fellow NATO member Denmark, raising fears of a rift that could destabilize the alliance.
