During the morning meeting with Stoltenberg, Trump also lambasted defense bloc allies and Germany in particular for giving billions of dollars to Russian Federation, a country the members "should be defended against".
He continued the attack Thursday, complaining that "Germany just started paying Russian Federation, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russian Federation".
Breaking from a carefully-scripted session to focus on Ukraine and Georgia, Trump "forcibly restated his position on wanting North Atlantic Treaty Organisation members to reach a 2 per cent spending target to a short a deadline", a source has told Reuters, referring to a North Atlantic Treaty Organisation goal.
Trump claims the U.S. is already spending the equivalent of four per cent of its GDP on defense, but experts say the actual figure is 3.57 per cent.
The US president accused Germany of only spending "a little bit over 1 percent" of its economic output on defence compared to the 4.2 percent spent by the US "in actual numbers".
The exchange was part of an uncomfortable day as anxious Western allies were subjected to the US president's "America first" approach. "They are a very strong people".
Back in the U.S., Democratic leaders Nancy Pelosi and Chuck Schumer issued a joint statement describing Trump's "brazen insults and denigration of one of America's most steadfast allies, Germany", as "an embarrassment". I think that's very inappropriate.
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Trump had opened the first day of talks in Brussels on Wednesday with a public diatribe against Germany, the second biggest state in the Western defense alliance, before the mood appeared to have calmed as the summit went into its second day, focusing on operations beyond Europe. I think he's right about the spending levels.
"United States spends too much". The president is also not the first leader to point to the impact of Nord Stream 2 on Europe, echoing complaints from Eastern European allies who note it would cut out transit countries such as Poland and Ukraine.
Peter Navarro, director of the White House National Trade Council, echoed Trump's rhetoric, telling Fox Business Network that "Germany is a tremendous problem, both for Europe itself, and for the United States in this sense".
But Trump has been more conciliatory behind the scenes, including at a leaders' dinner Wednesday. Larsen has written to Trump about how this fixation on the two per cent commitment demonstrates a willful misrepresentation of both NATO's mission and how it is funded.
Stoltenberg later said Trump had "very direct language" but "when it comes to the core message we actually all agree... we have to share the burden more fairly". "Because of that we can say that we can make our independent policies and make independent decisions".
Trump's next stop will be London, where he'll be meeting with Queen Elizabeth II and British Prime Minister Theresa May.