President Trump on Thursday scolded NATO allies for not paying their fair share for defense during a ceremony at the alliance’s new headquarters in Brussels.

“NATO members must finally contribute their fair share and meet their financial obligations,” Trump said, as other leaders looked on from the sidelines.

Returning to a refrain from his campaign, Trump pointed out that 23 of the 28 member nations are not meeting NATO’s target of spending 2 percent of their gross domestic product on defense.

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“This is not fair to the people and taxpayers of the United States and many of these nations owe massive amounts of money from past years — and not paying in those past years,” Trump said.

Trump did not explicitly endorse Article 5 of NATO’s founding treaty, which declares that an attack on any allied state is an attack on all. He only spoke generally of “the commitments that bind us together as one.”

The omission stood out further because the ceremony was held to unveil a memorial symbolizing the mutual-defense pledge: a twisted piece of metal from the north tower of the World Trade Center. The 9/11 attacks are the only time the alliance has invoked Article 5.

White House press secretary Sean Spicer said later that Trump is “fully committed” to the mutual defense of NATO allies.

“We’re not playing cutesy with this,” he told reporters. “I’ve seen some of the questions I’ve gotten from you guys, but there’s 100 percent commitment to Article 5.”

Trump's speech provoked exasperation from NATO leaders already uneasy with their relationship with the president, who condemned the alliance as “obsolete” during the 2016 campaign.

French President Emmanuel Macron stole bewildered glances with other leaders during Trump’s remarks. Trump appeared to take pleasure in lecturing his counterparts, even suggesting the NATO headquarters cost too much.

“I never asked once what the new NATO headquarters cost,” he said in an unscripted aside. “I refuse to do that. But it is beautiful.”

The speech was one of the more uncomfortable moments on Trump’s grueling, nine-day foreign trip. Trump received a hero’s welcome during previous stops in Saudi Arabia and Israel. But in Brussels, a city he once described as a “hellhole,” he was met with major street protests.

He was also put face-to-face with leaders he’s clashed with in the past, including German Chancellor Angela Merkel. Standing alongside Trump, the German leader spoke about a piece of the Berlin Wall that was being dedicated as part of the memorial.

“It is not isolation or the building of walls that will make us successful, but the sharing of values,” Merkel said in a subtle jab at Trump's immigration policies.

For all the awkwardness it has appeared to create, U.S. officials argue Trump’s approach has resulted in positive changes at the alliance.

After Trump’s repeated suggestions the alliance was not doing enough to fight terrorism, NATO announced it is officially joining the coalition against the Islamic State in Iraq and Syria (ISIS).

Trump tied the issue of defense spending to their commitment to the fight against terrorism.

“If NATO countries made their full and complete contributions, then NATO would be even stronger than it is today, especially from the threat of terrorism,” Trump said.

Updated: 1:48 p.m.