At the end of the first session of a two-day summit, NATO Secretary-General Jens Stoltenberg said the aim first was to reach 2 percent, but moments later Trump tweeted that allies were undercutting the United States on trade and needed to immediately boost spending.
During the trip, Trump has questioned the necessity of the alliance that formed a bulwark against Soviet aggression, tweeting after a day of contentious meetings: "What good is North Atlantic Treaty Organisation if Germany is paying Russian Federation billions of dollars for gas and energy?"
United States President Donald Trump on Tuesday expressed dissatisfaction with trade between the US and the European Union (EU), stating the latter makes it hard for USA's farmers and workers to do business in Europe. "Must pay 2 per cent of GDP immediately, not by 2025", he said.
In remarks later during the summit, however, President Trump increased pressure on allies over burden sharing, calling on allies to double defense spending targets to 4 percent, the White House confirmed.
The Brussels meeting comes less than a week before Mr Trump is due to hold his first summit with Vladimir Putin, in Helsinki, reviving concerns among USA allies over his proximity to the Russian president.
"These two people are very different, very interestingly different. But other presidents never did anything about it because I don't think they understood it or they just didn't want to get involved".
NATO Secretary General Jens Stoltenberg, German Chancellor Angela Merkel and US President Donald Trump talk during a family photo ahead of the opening ceremony of the NATO summit, at the NATO headquarters in Brussels, on July 11, 2018.
"We are not prisoners, neither of Russian Federation nor of the United States", Germany's Foreign Minister Heiko Maas told reporters.
Mr. Trump has spent weeks berating members of the alliance for failing to spend enough of their money on defense, accusing Europe of freeloading off the US and raising doubts about whether he would come to members' defense if they were ever attacked.
Stoltenberg said it had been appropriate and understandable for defence spending to fall after the end of the Cold War, but the alliance had now recognised the need to bring it back up, with member countries adding billions to their defence budgets. The United States pays about 67 percent of its gross domestic product on defense, more than any other nation, although only 3.5 percent is NATO-related. Energy exports represent Russia's main source of revenue, and Trump argues that the pipeline undermines NATO's goal, as it's created to counter Russian aggression.
Stoltenberg agreed there should be more done to prompt allies to invest more in their defense.
Mr Trump made more conciliatory remarks on Wednesday after he and Mrs Merkel met on the sidelines of the Brussels summit, saying defence spending and trade had been discussed.
Mr Trump, who landed in Belgium during the middle of the football-mad nation's World Cup semi-final match, will later head to London, where Prime Minister Theresa May's government is in turmoil over her plans for exiting the European Union.