In a startling public outburst, Trump told NATO Secretary-General Jens Stoltenberg that Germany was wrong to support a new $11-billion Baltic Sea pipeline to import Russian gas while being slow to meet targets for contributing to NATO defense spending that was meant to protect Europe from Russia.
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".
Speaking on the eve of the event, NATO Secretary General Jens Stoltenberg said: "Our Summit comes at a time when some are questioning the strength of the transatlantic bond".
"We have to talk about the billions and billions of dollars that's being paid to the country we're supposed to be protecting you against", Trump said, referring to European purchases of Russian natural gas.
The Nord Stream 2 project-an $11 billion extension of the existing Nord Stream natural gas pipeline by the Kremlin-backed gas giant Gazprom and five European nations-will double Russia's gas export capability to Germany to nearly 250 billion square feet per year.
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".
The EU is America's biggest export market, taking $501bn (£378bn) in goods and services in 2016, the latest year recorded by the Office of the US Trade Representative.
Mr Trump was expected to see German Chancellor Angela Merkel later in the day.
"We do have a lot of allies, but we can not be taken advantage of".
Mesut Ozil owes Germany fans Erdogan explanation - DFB chief Reinhard Grindel
Whilst German FA president Reinhard Grindel has called for Ozil to explain himself over the photo . He's booed and put up as a scapegoat - I completely understand that he is offended", he added.
Of Nato's 29 members, just five meet that target this year: the US, Greece, Estonia, the United Kingdom and Latvia.
As European Council president, Mr Tusk serves as the chairman when EU leaders meet at summits. 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.
Trump renewed his call for other North Atlantic Treaty Organisation allies to pay in more to the Western alliance after years in which US taxpayers have, he said, borne an "unfair" share of military spending.
"We're being taken advantage of by the European Union", Trump said, adding that US NATO spending "helps them a lot more than it helps us". "I've said that many times for many years".
Britain's summer 2016 referendum saw the leave side prevail by a 52-48 margin - a populist victory combined with Trump's surprise victory in the USA presidential race later in the year that sparked upheaval in the global order. "I think that getting along with Russian Federation, getting along with China, is a good thing".
"We're supposed to protect Germany but they're getting their energy from Russian Federation".
The comments will have made for uncomfortable viewing for the rest of Nato's 29 members, whose officials have voiced fears that Mr Trump could undermine the alliance during the gathering. "Just doesn't work!" he tweeted as he arrived in Brussels late on Tuesday.
"These countries have to step it up - not over a 10 year period, they have to step it up immediately", Trump said.
Trump will meet the Russian leader in the Finnish capital on July 16 for their first summit amid an ongoing investigation in the USA into possible Trump campaign collusion with Russia.