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 has set a bombastic tone for the talks, unleashing a barrage of criticism against European allies, including a clash with German Chancellor Angela Merkel.
Merkel, who grew up in communist East Germany, shot back that she had "experienced myself a part of Germany controlled by the Soviet Union, and I'm very happy today that we are united in freedom as the Federal Republic of Germany and can thus say that we can determine our own policies and make our own decisions and that's very good".
"We will see what happens - just a loose meeting". "I've said that many times for many years".
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".
"Many countries are not paying what they should", Trump said, seated across from a visibly rattled Stoltenberg.
He continued: "Nato is much stronger than it was two days ago".
On day two, leaders will welcome non-NATO partners including Afghan President Ashraf Ghani and Ukraine's Petro Poroshenko to the alliance's new glass-and-steel headquarters as they seek to focus on policy rather than politics.
"Many countries in North Atlantic Treaty Organisation, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made", he said, tweeting from the air en route to a key North Atlantic Treaty Organisation summit in Brussels.
"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.
Merkel said: "We had a very intense summit".
Germany, Europe's biggest economy, spends just 1.24%, compared with 3.5% for the US.
The spike in tensions comes as President Trump is preparing for his first-ever summit with Russian President Vladimir Putin in Helsinki, Finland, in less than a week, sparking worry among allies. But, several sources said, Trump instead reopened in strong terms his demand that other countries spend more immediately.
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