"Germany is a captive of Russia because it is getting so much of its energy from Russia", Trump said, taking particular aim at the proposed Nord Stream II gas pipeline, which he has previously criticised.
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".
US President Donald Trump has told NATO Secretary-General Jens Stoltenberg that Germany is "captive" to Russian Federation because it is importing energy from there. Their concerns had already set in ahead of Trump's arrival on the continent, following weeks during which the US President has increasingly and vociferously aired out his grievances against many of them.
The remarks, which came during his first engagement at the two-day gathering, reflect Mr Trump's anger that so few countries are meeting Nato's 2 per cent defence spending target.
"In terms of a government who's radically rethinking the nature of its committeemen to Europe, the British are the closest to the Trump administration on those issues", he said.
Trump said it was "very inappropriate" that the United States is contributing to the defence of Europe when one of its richest countries is supporting a major energy project with Moscow.
Referring to Monday's planned summit with Russian president Vladimir Putin in Helsinki, Mr Trump added: "And I have Putin".
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.
"We do have a lot of allies, but we can't be taken advantage of", he said. And a year ago, during his first visit to its Belgium headquarters, Trump initially declined to explicitly support the organization's defense agreement.
Germany now spends 1.24% of its GDP on defense, and has committed to hitting 2% by 2024, something that Trump, and other U.S. leaders before him, have tried to hasten.
Trump's criticism of the alliance has stood in stark contrast with his warm words for NATO's main adversary, Russia.
Italian Interior Minister Matteo Salvini even had some Trump-esque praise for Mr. Putin, whom he hoped to meet during a trip to Moscow for the Wold Cup final.
European officials have expressed hopes that North Atlantic Treaty Organisation members can paper over their differences but Tusk's salvo exposed the deep transatlantic divisions.
But Germany, Europe's biggest economy, spends just 1.24 percent of GDP on defence, compared with 3.5 percent for the US.
Trump said: "We're protecting Germany, we're protecting France, we're protecting all of these countries".
"So I think the secretary-general likes Trump".
During a rally last week in Montana, Trump vowed that he would "tell NATO: 'You've got to start paying your bills.'" The president also said that Americans were "the schmucks that are paying for the whole thing".
'You tell me if that's appropriate because I think it's not. There are no official events scheduled for his time here, though U.S. Ambassador to Britain Woody Johnson said the president would "spend some time preparing for Helsinki", USA Today reported.
The NATO summit is also being attended by leaders of so-called NATO partner states that are not members of the alliance.