NATO Secretary-General Stoltenberg later told reporters that Trump had used "very direct language" but that all NATO allies were agreed that the cost of defence spending must be spread around and that past year had seen the biggest increase in a generation.
The cost-sharing issue tops Mr. Trump's agenda for the North Atlantic Treaty Organisation summit.
Debunking Trump's claim ahead of Wednesday's meeting that the U.S.is "paying for 90 percent of North Atlantic Treaty Organisation", D'Angelo Gore of FactCheck.org noted, "In reality, the USA share of the commonly funded North Atlantic Treaty Organisation budget is now just over 22 percent, according to the most recent figures from North Atlantic Treaty Organisation".
Speaking at a signing ceremony for a new EU-NATO cooperation agreement, Mr. Tusk used the opportunity to send a personal message to Mr. Trump, who is due to arrive in the Belgian capital later on Tuesday.
According to a 2006 North Atlantic Treaty Organisation agreement, each country pledged to spend at least 2 percent of its gross domestic product on defense.
But Merkel nevertheless insisted Washington and Berlin remain allies, saying: "We are partners, we are good partners and wish to continue to cooperate in the future".
Trump's opening salvo took aim at Germany, one of many countries that he has argued aren't spending enough money.
Merkel responded directly to Trump's accusation in a press conference later on Wednesday.
"Instead, President Trump just insults friends who have sacrificed to hold that pressure together".
Opening the North Atlantic Treaty Organisation summit in Brussels with a bang Wednesday, President Trump blasted Germany for a pipeline project that he said made Germany "totally controlled by Russian Federation". "We pay far too much, and they pay far too little, but we will work it out, and all countries will be happy".
Trump said Germany was paying "billions and billions of dollars" to Russian Federation, "the country that we are supposed to be protecting you against", adding that North Atlantic Treaty Organisation had to look into this. We're protecting everybody. this has been going on for decades. "I think it's something North Atlantic Treaty Organisation has to look at". "I think it is something North Atlantic Treaty Organisation has to look at".
'It should never have been allowed to happen.
While on Air Force One en route to Belgium for a North Atlantic Treaty Organisation summit, President Trump took to Twitter to attack the alliance, going so far as to suggest he expects member nations that haven't spent enough on defense to reimburse the United States. Not fair to the US taxpayer.
"I think these countries have to step it up, not over a 10 year period, they have to step it up immediately". New Poll says Trump, at over 90%, is the most popular Republican in history of the Party.
Germany imports roughly 94% of its natural gas. "This embarrassment below. That's what", referring to a video of Trump's comments. Because of that we can say that we can make our independent policies and make independent decisions.
The president is also expected to meet with Russian President Vladimir Putin in Helsinki on Monday.