"We're supposed to be guarding against Russia and Germany goes out and pays billions and billions of dollars a year to Russia", Trump said in the presence of reporters at a pre-summit meeting at the residence of the USA ambassador to Belgium.
Trump has long complained that European Natomembers do not pay enough for their own defence, accusing them of freeloading on America and singling out Germany for particular criticism.
Meeting with NATO Secretary General Jens Stoltenberg over breakfast in the USA embassy in Brussels, Trump said Germany was "captive to Russia" and criticised its failure to raise defence spending.
Trump is to meet Putin in Helsinki next week in a summit that has generated much unease.
European allliance members were braced for criticism from Trump on defence spending, but his furious tirade at what should have been an amicable breakfast meeting appeared to take even North Atlantic Treaty Organisation chief Jens Stoltenberg by surprise.
"We're supposed to be guarding against Russia and Germany goes out and pays billions and billions of dollars a year to Russia", Trump said.
"I expect open and frank discussions, and there are disagreements on different issues", he said as he arrived at NATO's gleaming new headquarters for the summit.
The belligerent remarks were made at Trump's first engagement of the two-day summit in Belgium, as he sat for breakfast with NATO Secretary General Jens Stoltenberg. "I think that's very inappropriate".
Trump said that past presidents did not confront America's allies because they did not want to meddle in their affairs or they were blind to the problem.
Previous US presidents also urged greater defence spending from European nations.
The 29 allies are due to restate a commitment to try to spend two percent of GDP on defense by 2024, according to excerpts of their final summit declaration seen by AFP.
"It's certainly going to be an interesting time with NATO", Trump told journalists.
Both countries will be well-served in the meantime by the British parliament's recent decision to ratify the deal, known as CETA, said Joseph Pickerill, a spokesman for International Trade Minister Francois-Philippe Champagne. The vast undersea pipeline is opposed by the USA and some other European Union members, who warn it could give Moscow greater leverage over Western Europe. "He may be the only one but that's OK with me", the president said moments before the public rebuke of Stoltenberg.
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. "Frankly, Putin may be the easiest of them all", said Trump.
Mr. Tusk's challenge to the president appeared more reflective of the European Union than of Poland, where Mr. Trump is relatively popular and leaders are lobbying for a major US military presence in their country. "We're not going to put up with it, we cant put up with it", he said.
But European Council President Donald Tusk was more direct in anticipating that Trump may have designs on sowing discord, delivering a stinging warning to the visiting Americans president. At a time when Russian Federation is an worldwide pariah because of its invasions of neighbors like Georgia and Ukraine, the president of the United States has repeatedly denigrated the country's historical allies in Europe while expressing hope that relations with Russian Federation will become closer.
The prime minister said: "NATO is as vital today as it ever has been and our commitment to it remains steadfast".
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