In a series of tweets from the Belgian capital, Mr. Trump said, "Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more toward their protection from Russian Federation". "He may deny it", Trump said.
"I would say there might be an escalation between us and the Iranians".
"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations", White House press secretary Sarah Huckabee Sanders said in a statement.
The Nato leaders, gathered in Brussels, held an unplanned emergency meeting earlier on Thursday after Mr Trump told allies to immediately increase their defence spending, diplomatic sources said.
BRUSSELS-US President Donald Trump assured the US's readiness to return to the bargaining table if Iran initiates talks, he said in a statement at a North Atlantic Treaty Organisation news conference when asked about rising tensions with Iran on Thursday morning.
The U.S., which spent $685.9 billion on defense in 2017, now makes up 51.1 percent of NATO's combined GDP.
Mr Trump dominated the day and is set to stay in the spotlight as he visits the United Kingdom and prepares for a summit with Russian president Vladimir Putin.
In fact, Trump didn't just suggest he couldn't do anything to force the issue with Putin; he even seemed to acknowledge the possibility of concessions to Russian Federation.
"We're paying for anywhere from 70 to 90 percent to protect Europe, and that's fine."
"At no point did President Trump, neither in bilateral meetings nor in multilateral ones, say he would pull out of NATO", Macron said.
Fox Business' Brittany De Lea contributed to this article.
"What's more surprising, the fact that the President Trump is calling them out on that or that previous presidents haven't?" he asked.
Trump would normally first need to win approval in the Senate and to persuade Republican senators that the move was necessary and in the US's long-term security interests.
"Germany has agreed to do a lot better than they were doing, and we're very happy with that", Trump said, without going into specifics.
"And frankly maybe everyone is going to have a good relationship with Russian Federation". "And, frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them".
It was the most recent in a series of demands and insults that critics fear will undermine the decades-old alliance, coming days before Trump sits down with Putin at the conclusion of his closely watched European trip.
"I have to tell you that the atmosphere last night at dinner was very open, was very constructive and it was very positive, " Kolinda Grabar-Kitarovic, the president of Croatia, told reporters.
Amid the tumult, British Prime Minister Theresa May, whose government is in turmoil over her plans for exiting the European Union, sounded a call for solidarity among allies.
"As we engage Russian Federation we must do so from a position of unity and strength - holding out hope for a better future, but also clear and unwavering on where Russian Federation needs to change its behavior for this to become a reality, " she said.
"Germany is just paying a little bit over 1 percent, whereas the United States in actual numbers is paying 4.2 percent."