Trump threw the summit into turmoil by demanding not only that allies reach their commitment to increase spending to two percent of GDP "immediately" - instead of by 2024 as previously agreed - but also telling them to eventually double the figure to a punishing four percent.
Trump tweeted an hour before the second day of the summit got under way.
Mr. Trump has spent weeks berating 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 ever attacked.
Poland's President Andrzej Duda dismissed this fear, telling reporters he and Trump on Wednesday even discussed the USA increasing its military presence in Poland. Still, European economies have been growing recently, making the 2 per cent of GDP target ever harder to achieve, and Trump's spending demand is at least in part aimed at drumming up business for the USA defence industry. He said he believed in North Atlantic Treaty Organisation, but added that the European Union had treated the U.S. very unfairly on trade.
Mr Trump, who on Wednesday singled out Germany in a blistering attack on its record, said he had made clear in Thursday's emergency session that he was "extremely unhappy" with what had been on the table so far. "It's been wonderful to see the level of spirit in that room".
"Every time this president has had to push back on Russian Federation, he has", she argued, citing sanctions on Russian Federation, expulsion of diplomats, strikes on the Assad regime in Syria and a refusal to recognize the annexation of Crimea.
"Putin is not America's friend, nor merely a competitor", McCain said.
On Thursday, Trump reiterated his criticism of NATO in a tweet, stating, "Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more toward their protection from Russian Federation".
In an apparent swipe at Trump, he added: "Sometimes the corridors, comments and tweets take on more importance than what is negotiated, said or endorsed by heads of state".
French President Emmanuel Macron and Dutch Prime Minister Mark Rutte explained that it was not possible to mobilise funding outside the agreed national budgets.
"Secretary Stoltenberg gives us total credit, meaning me, I guess, in this case, total credit".
"There is a communiqué that was published yesterday".
"What you spend on a military budget doesn't necessarily automatically lead to greater capacity to contribute to NATO's defence", he said.
"It is up to President Trump to hold Putin accountable for his actions during the meeting in Helsinki", McCain said.
In an impromptu press conference this morning, Trump said he had "told people he would be very unhappy if they did not up their commitments very substantially".
Lithuanian President Dalia Grybauskaite told reporters she's comfortable with the Trump-Putin summit, but that it "depends very much on outcomes".
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.