He also said that his meeting with Russian President Vladimir Putin in Helsinki next week would be the "easiest" of the meetings he'll have while he's overseas.
"As far as I'm concerned, a competitor", he noted, adding that it would be good to get along with Russian Federation and China.
Referencing Trump's impending summit with Putin, Tusk on Tuesday said, "It is always worth knowing: who is your strategic friend?" "Boris Johnson's a friend of mine".
Defense spending among the 29 nations belonging to the transatlantic military alliance increased to an estimated 2.42% of gross domestic product (GDP), up from 2.4% in 2016, the organization said earlier this year.
At least one senior White House official has signed off on the North Atlantic Treaty Organisation summit statement, one North Atlantic Treaty Organisation diplomat said, which will contain a commitment to the 2 percent spending goal.
The meeting of Western leaders in Brussels has the potential to turn into another public bust-up following a divisive and bad-tempered summit of G7 nations in Canada in June.
"Dear America, appreciate your allies, after all you don't have that many", Donald Tusk said after signing a joint European Union-NATO declaration on cooperation with alliance Secretary-General Jens Stoltenberg in Brussels.
While Mr Trump is looking to improve relations with Russian Federation, there has been alarm at the deteriorating climate with his allies in Europe and what he might agree to with President Putin.
But unlike his recent tweets in which he berated allies, Trump struck a more affable tone in person before taking his flight: "We will work it out and all countries will be happy".
"The United States continues to devote more resources to the defense of Europe when the Continent's economy, including Germany's, are doing well and security challenges abound".
"NATO has been able to overcome those differences again and again".
But it adds that they will also "submit credible national plans on its implementation" in language that will perhaps please the USA president. "We're being taken advantage of by the European Union", he told reporters.
With the established global order on shaky footing, President Donald Trump's weeklong trip to Europe will test already strained bonds with some of the United States' closest allies, then put him face to face with the leader of the country whose electoral interference was meant to help put him in office.
Trudeau delivered that message during a day-long visit to Latvia ahead of the 29-country military summit, and just hours after Trump moved to disrupt the meeting by casting Russian Federation in a positive light. "Not fair to the USA taxpayer", Trump said. Allies are braced for a barrage of invective from Trump for not spending enough on defence, and are apprehensive that his often skeptical tone on the alliance that has underpinned European security for 70 years might turn into outright hostility.