The issue dominated the president's public remarks after he arrived in Brussels on Tuesday, striking fear among European officials that the president would question the US' role in the alliance.
"I think I can probably can [pull out of NATO], but that's unnecessary, and the people have stepped up today like they've never stepped up before", Trump added.
But this particular outburst, which took place at the start of the two-day North Atlantic Treaty Organisation summit in Brussels, has raised the stakes in his long-standing attacks on a critical alliance.
In 2017, the alliance saw its biggest increase in defense spending "in a generation", he said, signaling to his aggrieved ally in the White House that "we see that all Allies have started to increase defense spending based on the national plans we have agreed to develop a year ago". The US president has been particularly critical of Germany, accusing Berlin of relying on the US for security while not paying enough into NATO's coffers.
He also said meeting the queen would be an experience Trump "will really cherish".
When asked how that conversation might go, Trump said, "He may deny it".
Asked if Mr Putin presented a threat, Trump said, "I don't want him to be and that is why we have NATO". Because of that we can say that we can make our independent policies and make independent decisions.
Ian Wright Mocks Roy Keane's Irish Accent During On-Air Spat
Keane stuck to his original point, though, replying: "I don't mind you being happy, but you were getting carried away". Keane argued with former England striker Ian Wright and said: "Before the game, the whole talk was about the final".
"I don't think it should take a very long period of time and we will see where it leads but could lead to productive, something productive".
That would represent a massive upheaval of budgetary priorities in Europe where Germany and many others have pledged only to reach 2 percent by 2024 or later, and it was not clear what allies would spend the money on.
But with tensions in the Western alliance smouldering over Trump's trade tariffs on European steel and his demands for more contributions to ease the burden on U.S. taxpayers, his earlier remarks fuelled concerns among allies for the United States role in keeping the peace that has reigned since World War II. "All I can do is say it". But Trump's Thursday morning tweet, before meetings on Russian aggression and the war in Afghanistan, said allies "must" get to 4 percent. Though NATO allies have squabbled in the past, Trump seemed to be leaving in his wake not just bruised feelings, but deeper concerns about the future of the Western alliance that has served as a bedrock of the global order.
On both matters, Republican senators said they agreed with Trump's message. "And when you have that occurring, especially with Russia's activities, yeah, it undermines our alliance".
Trump said "actual" USA military spending is 4.2 percent of GDP, without explaining the big discrepancy.
Woody Johnson, the USA ambassador to the United Kingdom, dismissed the significance of the protests, telling Fox News that one of the reasons the two countries are so close "is because we have the freedoms that we've all fought for". "You know, he's done so well in his negotiations", he said, "I'm not about to second-guess how he's doing it".