In a sign of the rows which are expected to take place over the two day summit, Mr Trump questioned why Germany pays billions of dollars to Russian Federation for gas supplies but then demands that the U.S. defends it from Russian Federation.

Trump's criticism might put further strain on the relationship between the Western allies, especially following last month's turbulent G7 meeting.

Prior U.S. administrations have pressed Europe to invest more in its defense, but the matter has typically been handled by other officials in the government, said Ted Bromund, a senior research fellow at the Heritage Foundation.

"Nations were cutting defence spending by billions of dollars, now they are adding billions of dollars", he said.

After the two-day summit in Brussels, Trump will meet Russian President Vladimir Putin in Helsinki on Monday.

"Frankly Putin might be easiest of them all".

"I think it's inappropriate", he said repeatedly in an uncompromising conversation with Nato Secretary General Jens Stoltenberg.

And he vowed not to be "taken advantage" of by the European Union, which he accuses of freeloading by relying on the United States for its defence while restricting USA imports into the bloc, the world's biggest market. Trump's first words signalled this one is not going to be like this. They're also anxious about his potential embrace of Russia's Vladimir Putin during a summit in Helsinki.

With French and German officials urging member states to paper over their differences, EU President Donald Tusk delivered a blunt message to Trump yesterday, telling him "the U.S. doesn't have and won't have a better ally than the EU".

"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's comments are "frightful" even if he has a point about some North Atlantic Treaty Organisation countries not spending enough on defence, including Canada, said Brett Boudreau, a retired Canadian Forces colonel now based in London and who was formerly posted to North Atlantic Treaty Organisation headquarters in Brussels. "It is always worth knowing who is your strategic friend and who is your strategic problem", he said.

Before leaving Tuesday, Trump told reporters about this week's meetings, which he called interesting and said "all countries will be happy" with.

Mr Trump has threatened that he would only protect countries who met the two per cent defence spending obligation the alliance states agreed to meet by 2024.

"He chews out the allies then embraces the adversary", one diplomat told AFP recently.

Speaking to a cheering crowd at a rally this week, Trump told them that the United States would no longer be "the schmucks paying for the whole thing".

Looking for signs that the president remained committed to the alliance, Mr Stoltenberg pointed to a rise in American presence in Europe as well as a spending increase of 40 per cent on defence in Europe since the U.S. president's inauguration.

"I think it's very sad when Germany makes a massive oil and gas deal with Russia where - 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.