He claimed in a press conference that "Yesterday I let allies know I was extremely unhappy with what was happening".
Trump brushed off the protests, saying, "I think they like me a lot in the United Kingdom".
Trump had spent his time in Brussels berating members of the military 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 attacked.
But, several sources said, Trump instead reopened in strong terms his demand that other countries greatly speed up their progress toward a North Atlantic Treaty Organisation target agreed in 2014, of spending at least 2 percent of their GDP on defense to counter threats ranging from Russia's military modernization to militant attacks on European cities.
The summit's dramatic, chaotic conclusion - complete with the USA president suddenly making demands following a formal communique - bore a striking similarity to Trump's departure from last month's G7 meetings in Quebec.
Trump told reporters that allies had agreed to meet defense spending figures faster than previously planned, and said that financial commitments would increase in the future.
"And frankly, maybe everybody is going to have a good relationship with Russian Federation, so there will be a lot less problem with the pipeline", Trump said.
Still, Trump has been more conciliatory behind the scenes, including at a leaders' dinner Wednesday.
Reversing the harsh criticisms he has leveled at NATO, President Trump says the alliance is very strong - in part because of promises from America's allies to boost their military budgets to 2 percent of their gross domestic product. "He's been very nice to me".
Trump said he wanted "peace all over the world, that's my goal", and he wanted to achieve this by building up the military, with his dream to have the best military equipment in the world and never have to use it.
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".
With allies fearing he could strike some sort of deal with Nato's key adversary, Trump said that he saw Putin as a "competitor" but not an "enemy". "Are we contributing the kinds of resources and demonstrating the kind of commitment to the alliance that always needs to be there?" "I think that's very inappropriate".
U.K. Prime Minister Theresa May announced that Britain will be deploying an additional 440 troops to Afghanistan to help the country fight the Islamic State and the Taliban, nearly doubling its commitment to the NATO-led mission, called Resolute Support. "The United States has wasted $5.6 trillion on wars in Iraq and Afghanistan with nothing to show for it", she said.
When questioned in Latvia on his way to the summit in Brussels, Belgium, Trudeau said that Canada has no plans to double its defence budget when asked about meeting the two per cent figure directly.
When asked if he was anxious that the Putin-Trump meeting could have negative consequences for Poland, Duda said he expected Trump to continue the US stance of supporting Poland.
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