In direct costs, the US now pays about 22 percent of NATO's "principal budgets" that are funded by all alliance members based on a cost-sharing formula that factors in the gross national income of each country.
Member states are bracing themselves for more from the president in person.
"Wow!" And in multiple tweets, he reiterated a familiar theme, complaining that the USA is shouldering an unfair share of defense costs overseas: "NATO countries must pay MORE, the United States must pay LESS", he tweeted.
On the call with reporters, after talking about defense spending, Hutchison listed other objectives Trump is invested in at the summit, including a military readiness initiative, North Atlantic Treaty Organisation command structure reform and a new training mission in Iraq.
At the time, Johnson was Mayor of London and said Trump was talking "complete and utter nonsense".
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Trump, who said he has not spoken with May since the resignations, described Johnson as a "friend of mine" and said he might speak with him during the trip. "A competitor", the president said.
The United States outspends every country on the planet on the military, and its defense spending makes up more than two-thirds of NATO's indirect costs.
While it's true that only a handful of the 28 member countries now spend 2 percent or more of their GDP on defense, a 2014 agreement gave all of them a decade to get to the 2 percent level. "I've said that many times for many years".
Accusing the president of criticising Europe "almost daily", Mr Tusk said the European Union spent more than Russian Federation on defence and as much as China.
He added: "NATO countries must pay MORE, the United States must pay LESS".
"Well, we do have a lot of allies but we can not be taken advantage of, we are being taken advantage of by the European Union", he said. Trump has slapped tariffs on billions of dollars worth of steel and aluminium imports from China, the European Union, Canada and others, prompting retaliation against USA products. Trump vowed that European countries will not take advantage of the U.S., accusing them again of not spending enough on defence. "It would just be once more a transitory verbal tiff, as opposed to a change in policy, which leads to a true weakening of the alliance", says John Herbst, former USA ambassador to Ukraine, now at the Atlantic Council. Trump left that meeting without signing the joint statement from the seven nations and antagonizing important US allies, including Canadian Prime Minister Justin Trudeau.
A country's defense contribution shouldn't be measured only as a percentage of GDP, but also in terms of troops and hardware, she said in an interview with Bloomberg Television in Berlin.