Asked for comments on Tusk's remarks, Trump said that the United States did "have a lot of allies".
Mr Stoltenberg was careful not to provoke the U.S. president ahead of a potentially testy meeting.
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.
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But Trump countered by saying the arrangement puts the United States at a disadvantage, pointing to the measly share of gross domestic product (GDP) that most members spend on defense.
Before taking off on Air Force One, he tweeted: "Getting ready to leave for Europe".
This week's two-day summit, beginning on Wednesday at Nato's new headquarters in the Belgian capital, is expected to be dominated by USA demands for increased spending by other members of the 29-nation alliance.
He is expected to repeat his demands for more military spending on Wednesday (Thursday NZT).
Speaking about his European trip, Trump said his sitdown with Putin might be "easiest" of all the meetings. "Europe far more than it does the US".
"Dear America, appreciate your allies, after all, you don't have that many", Tusk said. "NATO has not treated us fairly but I think we will work something out".
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I think he was away at an England camp and he had to fly back to see the birth of his child, he got there 10 minutes before - so hopefully I can do the same".
Though the European allies expressed dissatisfaction with the uncertainty that Trump's approach has brought them, they appear to be reluctantly getting onboard. Hodges said a negative stand on Germany was unfair.
Britain's Prime Minister Theresa May arrives on the continent wounded by the high-profile resignations of two senior cabinet members and Brexiteers: David Davis and Boris Johnson. It will deal with current security challenges, including the relation with Russian Federation, the sharing of defence expenditures, the modernisation and reform of the North Atlantic Treaty Organisation command structure and NATO's further enlargement.
Trudeau, who was called "weak" by Trump after the G7 meeting, pledged to boost Canadian troop numbers 540, from around 450 now.
Trump replied, "Well, we do have a lot of allies, but we can not be taken advantage of".
Nervous about Russian Federation since Moscow's 2014 annexation of Crimea, POLAND and ESTONIA also sit right at the 2 percent mark, while LATVIA, LITHUANIA and ROMANIA are set to join the group this year, according to government plans for 2018.
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