The feud over defense spending looks set to overshadow other business at the two-day summit.
U.S. President Donald Trump and U.S. First Lady Melania Trump attend the evening reception and dinner at the 2018 NATO Summit. White House spokeswoman Sarah Sanders said this is not a formal proposal and that Trump had merely "suggested" this increase.
According to Politico, Trump warned his allies behind closed doors that they would need to radically increase defense spending or the U.S. "will do our own thing".
"They have substantially upped their commitment and now we're very happy and have a very, very powerful, very, very strong NATO, " Trump told reporters shortly before leaving Brussels for London to meet with British Prime Minister Theresa May and Queen Elizabeth.
"Germany is a captive of Russia because it is getting so much of its energy from Russia", Trump said, taking particular aim at the proposed Nord Stream II gas pipeline, which he has previously criticised. He added: "The people have stepped up today" as they never have before.
The populist core of his coalition is fueled in part by anger over what his supporters consider unfair treatment of the United States on matters of trade, immigration, and worldwide affairs.
"He likes the ideas of their leaders being destabilized and therefore easier to manipulate for him", said Tim O'Brien, a Trump biographer.
Trump has explicitly linked North Atlantic Treaty Organisation with a transatlantic trade row, saying the European Union shuts out United States business while expecting America to defend it. I think they agree with me on immigration.
"I don't. I'm very consistent".
At NATO, Trump did not specify which countries had committed to what, and it remained unclear whether any had changed their plans. That would mark a significant milestone for the alliance. "But in each of the last 4 years, the vast majority of European nations in the alliance have increased their defense spending as percentage of GDP".
Trump had "reaffirmed his commitment to a strong NATO".
Asked when exactly the allies would now reach their two percent of GDP target, Trump said it would be over the coming years.
But Macron said no direct threat was issued.
"I let them know that I was extremely unhappy", he said, but added that the talks ended on the best of terms: "It all came together at the end".
The summit in Brussels is shaping up as the alliance's most hard in years, against a backdrop of growing unease about the threat from Russian Federation and deepening transatlantic tensions in fields ranging from trade to energy. Citing the alliance's Secretary-General Jens Stoltenberg, he said: "Secretary Stoltenberg gives us total credit, meaning me, I guess, in this case, total credit".
Merkel told reporters there followed a discussion with assurances to Trump that spending was increasing - something he later acknowledged was happening at an unprecedented rate.
As well as Merkel, Spanish Prime Minister Pedro Sanchez and Charles Michel, the prime minister of Belgium, were singled out by Trump for undershooting on their spending targets when USA taxpayers, funding a defence budget worth about 3.6 percent of their national income, foot much of NATO's bills. "The U.S. pays tens of Billions of Dollars too much to subsidize Europe, and loses Big on Trade!"
He has also accused the country of being held prisoner to imported Russian energy, saying it is "totally controlled" by Moscow.
Trump's surprise demand came after he clashed with Chancellor Angela Merkel, calling Germany a "captive" of Russian Federation because of its gas links and singling out Berlin for failing to pay its way.
"Not acceptable!" he railed before a day of events that will include meetings with the leaders of Azerbaijan, Romania, Ukraine and Georgia before he heads to his next stop: the United Kingdom.