Trump has berated members of the alliance for failing to spend enough of their money on defense, accusing Europe of freeloading off the US and raising doubts about whether he would come to members' defense if they were attacked.
A White House spokeswoman said his remarks came as he was urging leaders to increase their outlays on defence and were not a formal proposal.
Officials said that non-members of the alliance had been asked to leave the room early Thursday and that everyone in the room had been told to leave their phones outside.
"NATO is much stronger now than it was two days ago", Trump told reporters, describing an unscheduled crisis meeting of the 29 alliance leaders on Thursday morning as "fantastic" and having "a great collegial spirit".
Trump arrived on the back of a barrage of criticism of Europe on issues ranging from trade to energy and above all his claims that the continent freeloads on the back of America for its defence. "We're protecting everybody. And yet we're paying a lot of money to protect". "Germany is a captive to Russian Federation because it is getting so much of its energy from Russian Federation".
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He said that German Chancellor Angela Merkel runs the nation that is the "de facto leader of the EU" and that its failure to "properly fund NATO" should be criticized. Trump tweeted an hour before the second day of the summit got under way.
The President said: "We are supposed to be guarding against Russian Federation and Germany goes out and pays billions and billions of dollars a year to Russian Federation". A source close to Macron said Trump had voiced his "personal attachment" to Europe and gave "rather positive and constructive messages" to his allies. "We are talking about trade, we have a very, very good relationship with the chancellor, we have a tremendous relationship with Germany", Trump said with Merkel sitting at his side. "Since Trump became president, US funding for military presence in Europe - the European Deterrence Initiative -has been increased by 40 percent". So I think the secretary-general likes Trump.
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 previous year".
"We call upon [North Korea] to fully implement its global obligations; to eliminate its nuclear, chemical, and biological warfare capabilities and ballistic missiles, and abandon all related programmes", the statement said.
Despite his continued criticism of the transAtlantic pact, Trump on Wednesday signed the 23-page North Atlantic Treaty Organisation declaration, along with his 28 other allies, which reaffirmed the two percent goal to be reached by 2014.
U.S. officials have told Reuters that Washington is preparing another review of strategy, a year after Trump begrudgingly agreed to extend involvement in the 17-year-old war.