Trump has said he wants to focus on fighting Islamic terrorism and, in a brief encounter with the Belgian prime minister, referred to a suicide attack claimed by Islamic State that killed 22 people in Manchester on Monday.
Daalder says Secretary General Jens Stoltenberg and other NATO leaders are going to point to a commitment they made in 2014 - a requirement that all NATO countries put 2 percent of their GDP into NATO defense spending by the year 2024 - as proof that they're working towards that goal.
"NATO members must finally contribute their fair share and meet their financial obligations", Trump said.
"Twenty-three of the 28 member nations are still not paying what they should be paying and what they're supposed to be paying for their defence".
US President Donald Trump called on North Atlantic Treaty Organisation members to focus on defeating terrorism and urged them to pay a fair share towards the Alliance's upkeep.
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Majority carry the Belgian flag and "We fight against Donald Trump and his billionaire's friends" banners, "Stay away from our gully" or "Mr. Trump, you are not welcome here".
"All people who cherish life must unite in finding, exposing and removing these killers and extremists and yes, losers, they are losers".
"I think you can expect the president to be very tough on them, saying, 'Look the USA is spending 4 percent".
Leaders had hoped for more, although a White House official insisted Trump, by being a member of the alliance, supported NATO's collective defence clause, which stipulates that an attack on one ally is an attack against all.
"My main message to President Trump was that what gives our cooperation and friendship its deepest meaning are fundamental Western values, like freedom, human rights and respect for human dignity", Tusk said.
Trump is slated to leave Brussels late Thursday for the final piece of his trip, a two-day stay in Sicily for G-7 meetings.
As he met Macron Trump pointedly refused to answer journalists' questions about the growing intelligence scandal, which comes on top of concerns his administration may not have handled secret information provided by allies with the due care expected. The leaders also took part in an event to unveil memorials for the tragic events of September 11, 2001, and the historic fall of the Berlin Wall in November 1989.