President Trump, speaking at a ceremony Thursday to dedicate a memorial to NATO’s resolve in the wake of the 2001 terrorist attacks on the United States, publicly chastised his fellow alliance leaders for not being “fair” to U.S. taxpayers.

Trump used the occasion of his maiden summit with NATO leaders in Brussels, where he was invited to dedicate the September 2001 memorial, to remind the alliance that “23 of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying,” and that they owe “massive amounts” from past years.

European leaders who gathered from across the alliance gazed at Trump without expression and offered modest applause at the end of a speech that he began by asking for a moment of silence in remembrance of the victims of Monday’s terrorist attack in Manchester, England, that killed 22 and wounded many more.