May 25 (UPI) -- President Donald Trump visited NATO headquarters in Brussels on Thursday, telling top European Union officials that many member nations must spend more on defense.
"Members of the alliance 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 are supposed to be paying for their defense. This is not fair to the people and taxpayers of the United States."
All 28 NATO parties are asked to spend 2 percent of their gross domestic product on their own defense -- a measure intended to ensure all members can defend themselves, and therefore are less likely to rely on the alliance for aid. Some nations, though, have failed to meet that threshold.
The United States, which has the largest GDP of all member states, accounts for three-quarters of all NATO defense spending.
"Two percent is the bare minimum for confronting today's very real and very vicious threats," Trump said. "If NATO countries made their full and complete contributions, then NATO would be even stronger than it is today."
NATO Secretary General Jens Stoltenberg displayed a chart showing how the burden sharing has improved.
During the meeting, Trump did not restate commitment to NATO Article 5 -- a provision of membership that says an attack on one member state is an attack on all member states. European leaders have expressed concern about the United States' dedication to the bylaw and some had been hoping to hear support of it from Trump.
In nearly 70 years of existence, Article 5 has only been invoked once -- Sept. 11, 2001 -- and was the catalyst for NATO's involvement in the war in Afghanistan that followed.
Tension over U.S. support for Article 5 began during Trump's campaign for president, when he repeatedly threatened to militarily support only members who meet the 2 percent spending benchmark.
Counter-terrorism and other issues were also on the agenda for the one-day summit.
After a meeting with Trump, European Council President Donald Tusk said the leaders "discussed foreign policy, security, climate and trade relations."
"My feeling is that we agreed on many areas. First and foremost, on counter-terrorism, and I am sure that I do not have to explain why. But some issues remain open, like climate and trade," he told reporters.
Tusk said he is not "100 percent sure" that he and Trump "have a common opinion about Russia" but said they share the same beliefs about the conflict in Ukraine.
"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. "The greatest task today is the consolidation of the whole free world around those values, and not just interests."
During his visit Thursday, Trump also met with Belgium's King Philippe, Queen Mathilde and Prime Minister Charles Michel at the royal palace
Trump also met with recently elected French President Emmanuel Macron at the U.S. ambassador's residence in Brussels as part of a working luncheon.
"It is my great honor to be with the newly elected president of France, who ran an incredible campaign and had a tremendous victory," Trump said. "We have a lot to discuss, including terrorism and other things."
Macron said he spoke with Trump about the Paris agreement to reduce greenhouse gases. During his campaign, Trump said he would cancel the climate agreement and stop all U.S. payments to U.N. global warming programs.
"As for climate, well, President Trump can assert his position," Macron said after Thursday's meeting. "I respect the fact he has reviewed the Paris agreement. I reiterated the importance of the agreement."
Trump attended a working dinner with all 28 leaders of NATO countries after participating in a ceremony inaugurating the new $1 billion NATO headquarters in Brussels.
Following the meeting with NATO leaders, Trump and first lady Melania Trump will depart for Taormina, Italy, to attend a G7 summit.