Trump was referring to a 2015 agreement between Russian gas giant Gazprom and Germany to build a pipeline under the Baltic Sea.
"NATO is the most successful alliance in history".
Trump has harshly criticized May, Merkel and Trudeau since taking office and opened Wednesday with another broadside against Merkel, asserting that her country is "totally controlled" and "captive" to Russia as he objected to a deal to bring Russian natural gas directly to 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.
"Russia continues to say they had nothing to do with Meddling in our Election!", he tweeted last month before tearing into the Federal Bureau of Investigation and its former director James Comey.
That doesn't mean there haven't been ups and downs over the decades - seen dramatically after the September 11, 2001 attacks on the USA when more than 200,000 Germans took to the streets of Berlin to show their solidarity with the US, only to give way a year later to massive demonstrations against the American march toward the invasion of Iraq.
NATO's Stoltenberg later told reporters that Trump, who will meet Russian President Vladimir Putin on Monday, had used "very direct language" but that North Atlantic Treaty Organisation spending was rising strongly. German Chancellor Angela Merkel has said her government will increase spending to 1.5% of GDP by 2024.
He saidthey discussed defense investment and trade, adding: "We have a tremendous relationship with Germany".
"I myself have also experienced a part of Germany being controlled by the Soviet Union", she said.
"We're paying for anywhere from 70 to 90 percent to protect Europe, and that's fine."
Mary Civiello, an executive communications coach and expert on body language, told The Washington Post that Kelly's expression during Trump's comments seemed to suggest "he wants to be anywhere but where he is". "I think it's very unfair to our country".
U.S. President Donald Trump and U.S. First Lady Melania Trump attend the evening reception and dinner at the 2018 NATO Summit. Also present was France's President Emmanuel Macron, who tried this year to charm Mr Trump into more harmonious transatlantic relationship, only to see the U.S. president withdraw from the 2015 Iranian nuclear deal and impose tariffs on European goods. "Will they reimburse the USA?"
Meanwhile, President Trump is singling out Germany for not paying its fair share on defense while dolling out hundreds of millions of dollars to Russian Federation for energy. Members have pledged to get to 2 percent, but most countries are well off the mark - something Trump campaigned to change.
How did leaders respond to Trump's tirade? The gas pipeline mirrors the Nord Stream route going through exclusive economic zones and territorial waters of five countries - Russia, Finland, Sweden, Denmark, and Germany.
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