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. I think they agree with me on immigration.
"We have a very powerful, very strong North Atlantic Treaty Organisation, much stronger than it was two days ago", he said.
Officials at the meeting said Trump shocked many present and broke with diplomatic protocol by addressing German Chancellor Angela Merkel by her first name, telling her: "Angela, you need to do something about this".
"What they're doing is spending - at a much faster clip they're going up to the 2 percent level", Trump said. Trump also prepared to make a previously unscheduled public statement, the White House said.
A frustrated Trump has already hinted he may accept Russia's annexation of Crimea as he accepted Israel's annexation of Jerusalem.
McCain then ticked off a series by transgressions by Putin, including the "attack" on the USA election and efforts "to undermine democratic institutions throughout the world".
Leaders had gone into the second day, set to focus mainly on ending the long war in Afghanistan, with low expectations, showing the extent of European frustration with Trump's "America first" policies among the other 28 members of the alliance. In reality, 50% to 75% of Germany's natural-gas imports come from Russian Federation, and gas makes up less than 20% of Germany's overall energy mix.
"Ultimately, that will be going up quite a bit higher than that", he said.
Donald Trump's Jekyll and Hyde performance on the opening leg of his European tour - one minute angrily wielding a wrecking ball and the next playing the oleaginous anglophile - will hardly leave European diplomats, especially those in Britain, reassured as they brace themselves for his bilateral summit with Vladimir Putin on Monday.
"Allies would be increasing spending by $33 billion or more", he added.
"So I would imagine that's what they will do, but maybe they will take a little bit of a different route. Every single USA president has been pushing the point of extra spending since Reagan and they have all been played I am not going to be".
Trump has spent weeks berating members of the alliance for failing to spend enough of their money on defence, accusing Europe of freeloading off the US and raising doubts about whether he would come to members' defence if they were ever attacked.
But by Thursday, leaders from France, Germany and Italy offered no support for Trump's claim that he had wrung new concessions out of the alliance on their military spending. "The current commitment is to reach two percent by 2024 but with get-out terms that would allow some to stretch it out to 2030". However, EU officials claim that "while USA defense spending makes up 70 percent of combined allied governments' military budgets, just 15 percent of US expenditure is spent in Europe on NATO-related defense".
"We understand your message, but some people ask themselves, will you be tweeting differently once you're aboard Air Force One?" a Croatian reporter asked.
"He's been very nice to me the times I've met him".
Trump later called the meeting a "success".
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". "Very unified, very strong, no problem".