The summit in Brussels is shaping up as the alliance's most hard in years, against a backdrop of growing unease about the threat from Russian Federation and deepening transatlantic tensions in fields ranging from trade to energy.
Later Mr Trump and German chancellor Angela Merkel had a face-to-face meeting, where the row continued.
President Trump publicly criticized Germany over its Baltic Sea gas pipeline deal with Russian Federation, calling the wealthiest nation in the European Union "a captive to Russian Federation".
"We're supposed to be guarding against Russian Federation and Germany goes out and pays billions and billions of dollars a year to Russian Federation".
Trump has taken an aggressive tone during the summit, questioning the value of a military alliance that has defined decades of American foreign policy, torching an ally and proposing a massive increase in European defence spending. Must pay 2 per cent of GDP IMMEDIATELY, not by 2025.
Former secretary of state John Kerry blasted President Trump's criticism of Germany in a lengthy statement on Wednesday morning as "strange", "counterproductive", and "disgraceful". EURACTIV broke the story.
"NATO is not a stock exchange where you can buy security".
Trump took to Twitter to say publicly what he told North Atlantic Treaty Organisation leaders privately on Wednesday, calling on all allies to meet a commitment agreed in 2014 to spend 2 percent of economic output on defense to counter threats ranging from Russia's military modernization to militant attacks on European cities.
German Chancellor Angela Merkel appeared to hit back at US President Donald Trump's claim that "Germany is a captive of Russia" by drawing on her own upbringing in Soviet-controlled East Germany.
Diplomats were already anxious about the summit ahead of time, not least because of an abrasive G7 meeting last month, when Mr Trump renounced a summit communique that had previously been jointly agreed.
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Company shares fell as much as 5.9 per cent on Wednesday to US$47.80 near midday, the lowest intraday level since February 2016. The city of Jeffersonville, Ind., had plenty of reason to add John Schnatter's name last year to its 80-year-old gymnasium.
‘Strange and counterproductive’: Kerry slams Trump’s remarks at North Atlantic Treaty Organisation summit
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".
The US leader has also singled out Germany for particular criticism over its defence spending.
Germany, Europe's biggest economy, spends just 1.24%, compared with 3.5% for the US.
The United States and Canada swung sharply on Sunday (10 June) toward a diplomatic and trade crisis as top White House advisers lashed out at Canadian Prime Minister Justin Trudeau a day after US President Trump called him "very dishonest and weak".
"We had a fantastic meeting at the end".
There have been fears that Trump, keen to be seen to make a breakthrough with the Kremlin strongman, might make concessions that would weaken Western unity over issues like Ukraine and Syria.
There are flashpoints between the two leaders on a series of fronts - including Mr Trump's jibe that Britain is "in turmoil" and praise of Boris Johnson after his resignation over the PM's Brexit plans.
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.
British Prime Minister Theresa May tried to set the tone on Wednesday by announcing more troops for NATO's Afghan training mission.
NATO Secretary-General Jens Stoltenberg said, "We should first get to two percent", adding that eight of the 29 allies were meeting that target.
But Trump has been more conciliatory behind the scenes, including at a leaders' dinner Wednesday. I was 7 years old at the time.