Tusk issued a stern warning to Trump Tuesday as he signed a joint statement on cooperation between the European Union and NATO, "Dear America, appreciate your allies, after all you don't have that many".
Trump was referring to a 2015 agreement between Russian gas giant Gazprom and Germany to build a pipeline under the Baltic Sea.
As European Council president, Mr Tusk serves as the chairman when EU leaders meet at summits.
As cameras left the room, Trump assured reporters his tough tone will encourage North Atlantic Treaty Organisation members to pony up: "I have great confidence they'll spend more".
While countries closer to Russian Federation, such as Latvia, Lithuania, Romania, and Poland, significantly boosted defense, most other members took a more relaxed approach, with Germany's defense minister saying in 2015 that she didn't see a need to meet the spending bar, Deutsche Welle reported. "You tell me if that's appropriate".
At the pre-summit breakfast meeting he launched straight back to his theme.
United States ambassador to NATO Kay Bailey Hutchison urged allies to look beyond Trump's rhetoric and focus on the summit declaration - which the U.S. is expected to back - which will be the basis for the alliance's work over the coming years.
However, his comment that "Germany is totally controlled by Russia because they are getting 60 to 70 percent of their energy from Russia and a new pipeline" appeared to mis-state German energy use - about 20 percent of which is accounted for by oil and gas imports from Russia.
Some NATO members point out that only 15% of America's defence spending goes on defending Europe, and that the emphasis should be on adapting NATO for new threats, not simply spending more money on defence.
Many US presidents have urged European governments to spend more on their militaries.
May's relationship with Putin is frosty - the United Kingdom believes the Kremlin was behind the order to poison former spy Sergei Skripal in western England with a nerve agent - and the one with Trump could be better. "This is serious. My task is to try to minimize the negative impact on NATO", Stoltenberg told a forum in the margins of the summit.
"I'm very glad that today we are united in freedom. because of that we can say that we can make our independent policies and make independent decisions".
"That's a fundamental inconsistency in the pro-NATO position", Mr. Desch said.
German defence minister Ursula von der Leyen also responded defiantly to Mr Trump's barb.
The NATO chief was frank about the impact of Trump's criticism on the Western allies at a broader level and he referred to non-NATO issues such as trade, where Trump is angry over the US trade deficit with the European Union. "I want to compliment the secretary general, he's worked very hard on this problem". 'Germany is a rich country.
But Germany, Europe's biggest economy, spends just 1.24 percent of GDP on defence, compared with 3.5 percent for the US. Trump's trade adviser Peter Navarro went as far as telling a USA talk show that there was "a special place in hell" for Trudeau - a remark he later walked back. "I think that getting along with Russian Federation, getting along with China, is a good thing".
Mr Trump predicted as he departed Washington that the "easiest" leg of his journey would be his scheduled sit-down with Mr Putin.