During the summit, Trump reportedly broke diplomatic protocol by calling German Chancellor Angela Merkel by her first name, saying "Angela, you need to do something about this", a source told Reuters.
The summit in Brussels is shaping up as the alliance's most hard in years, against a backdrop of deepening transatlantic tensions in fields ranging from trade to energy and defence.
"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations", White House press secretary Sarah Huckabee Sanders said in a statement.
"This year, since our last meeting, commitments have been made for over $40 billion more money spent by other countries."
Trump is due to meet Putin on Monday in Helsinki.
"Since Trump became president, US funding for military presence in Europe - the European Deterrence Initiative -has been increased by 40 percent".
Trump arrived on the back of a barrage of criticism of Europe on issues ranging from trade to energy and above all his claims that the continent freeloads on the back of America for its defence.
North Atlantic Treaty Organisation leaders try to focus on Afghanistan and Ukraine following Trump row
And I think that leaders are also looking forward to your thoughts about the meeting with President Putin later on". The organization is asking all member states spend a minimum 2 percent of their GDP on defensive measures by 2024.
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As a strong proponent of diplomacy, I believe a meeting between President Trump and President Putin could be a productive one. The Guardian quotes her as saying, "I have experienced myself how a part of Germany was controlled by the Soviet Union".
He complained the United States "pays tens of Billions of Dollars too much to subsidize Europe" and demanded that members of the military alliance meet their pledge to spend 2 percent of GDP on defense, which "must ultimately go to 4%!" Trump said Germany agreed to accelerate its defense spending increases. The U.S.is paying for Europe's protection, then loses billions on Trade.
The tone of their remarks contrasted with an earlier breakfast between Trump and North Atlantic Treaty Organisation chief Jens Stoltenberg, when the Republican president said Germany's reliance on Russian energy left it "in the control" of Moscow and Merkel later referred to her youth in Soviet-run East Germany to insist Berlin was now fully sovereign.
President Trump's hard-line trade advisor Peter Navarro piled on Germany for its trade practices hours after the president blasted the longtime USA ally as 'totally controlled by Russian Federation'.
"We didn't know what to expect, it was a positive outcome", he said of the dinner in a Brussels park, where leaders were treated to an acrobatic display and saxophone music.
"We made a tremendous amount of progress today", he said. But at the bilateral meeting he exulted: "We have a very, very good relationship with the Chancellor".
Dr. Claudia Major, a researcher at the German Institute for International Security affairs, said that Germany, in particular, does more for the alliance than spend on its own defense, including deploying troops to Afghanistan, "and eventually, two percent is an arbitrary measurement that does not say much about efficiency and output". But, several sources said, Trump instead reopened in strong terms his demand that other countries spend more immediately.
"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. We have a fundamental disagreements on principles and objectives between Trump and the Europeans.