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BRUSSELS — President Donald Trump slammed fellow NATO leaders for not meeting financial obligations in a public broadside Thursday that surprised seasoned observers.

Some world leaders appeared surprised or bemused as the president said 23 of the 28 member states owed “massive amounts of money” to U.S. tax payers. Trump was unveiling a piece of the World Trade Center as part of an inauguration ceremony of NATO’s new headquarters when he criticized his fellow leaders for their lack of contributions to the alliance.

French President Emmanuel Macron appeared to be cringing while the Prime Minister of Luxembourg Xavier Bettel raised an eyebrow in response.

Trump said “2 percent was the bare minimum” countries should be contributing, referring to the 2 percent of GDP NATO advises its members to spend on defense. Most countries, with the exception of the United States, Britain, Estonia, Greece and Poland, fall short of this target.

Referring to the new NATO headquarters building, Trump said “I never asked what it costs, but it’s beautiful.”

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German Chancellor Angela Merkel also unveiled a part of the Berlin Wall that once separated East and West Germany.

Trump went on to call for a renewed and united effort to fight terrorism, referring to militants as “losers” who must be driven out of societies and “never ever let back in.”

He said the attack this week on concertgoers in the British city of Manchester showed the evils of terrorism. The president then called for a minute of silence for the victims.

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After the September 11, 2001, attacks, the U.S. triggered NATO’s collective defense clause, known as Article Five, prompting alliance members to rally around their ally. It is the first and only time the clause has been invoked.

The unveiling of the World Trade Center memorial was meant to be symbolic of the United States’ commitment to the alliance — but Trump’s failure to mention Article Five left commentators doubtful.

Trump's trip had been a relative success until now. It is now a failure. Close to a disaster unless he fixes it later today 3/3 — Tom Wright (@thomaswright08) May 25, 2017

Seasoned journalists were openly surprised at his failure to highlight his commitment to the alliance, while Thomas Wright, a fellow at the Brookings Institution think tank, tweeted “Trump’s trip had been a relative success until now. It is now a failure. Close to disaster unless he fixes it later today.”

White House Press Secretary Sean Spicer later confirmed to reporters that "there is 100 percent commitment to Article Five."

Another moment from the ceremony quickly went viral, after a video captured Trump appearing to shove another NATO leader, Prime Minister of Montenegro Dusko Markovic, out of the way.

Following the ceremony, the 28-member group was due to meet in the new headquarters where the main issues on the agenda were counterterrorism and sharing the burden of NATO’s budget.

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NATO leaders were due to give the go-ahead for the alliance to join the U.S.-led coalition against ISIS, which will be seen as a victory for Trump who has been pushing for more to be done to fight terrorism.

Following the meeting, Trump will leave Brussels for the Italian island of Sicily where he will attend the G-7 summit.

Sicily is the last stop on his first international trip as president, which has included visits in Riyadh, Jerusalem, Bethlehem and the Vatican.