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". He described US allies in the region as "afraid about what our president might agree to", and he told reporters he "very much" shares their worries.
As to whether he could unilaterally pull the US out of the alliance without congressional approval, Trump said, "I think I probably can, but that's not necessary".
Trump had kicked off the North Atlantic Treaty Organisation summit at breakfast with the organization's leader on Wednesday by accusing Germany of being "totally controlled" and "captive to Russia" because of a pipeline deal that would bring gas directly between the two nations.
Trump said he had been "extremely unhappy" with allies' low levels of defense spending before the summit, but was pleased "they have substantially upped their commitment" during a "fantastic" unscheduled crisis meeting on Thursday morning.
TRUMP: "Frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them".
On both matters, Republican senators said they agreed with Trump's message.
Europe's biggest economy is on course to spend just 1.24% of GDP on defense in 2018. Allies would be increasing spending by $33 billion or more, he added.
He complained the United States "pays tens of Billions of Dollars too much to subsidize Europe" and demanded that member nations meet their pledge to spend two per cent of GDP on defence, which "must ultimately go to 4%!"
USA ambassador to NATO Kay Bailey Hutchison told reporters that Trump would also recommit to one of the founding articles of NATO - Article 5 - which holds that an attack on one member is an attack on them all.
Trump's next stop will be London, where he'll be meeting with Queen Elizabeth II and British Prime Minister Theresa May.
German news agency DPA, Politico and The Times of London reported the US President had openly threatened that the US would "do our own thing" unless the wealthiest allies were spending 2 per cent of their GDP on defence by January.
Focusing on numbers, rather than capabilities, "is an inadequate way to measure allies' impact and their various contributions to NATO's mission of collective defense".
But French President Emmanuel Macron told reporters the alliance had not agreed to anything new during the meeting.
Mr Trump said he didn't want Russian Federation to be a threat, while adding that his meeting with Mr Putin next week "may be the easiest on his Europe trip".