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Addressing member states on Thursday, he spoke about the unveiling of a piece of the North Tower from the 9/11 attacks and a part of the Berlin wall, which has gone on display at the new building.

He then blasted the group as he joked “I never asked, once, what the new Nato headquarters cost – I refuse to do that.”

He said: “I want to extend my appreciation to the 9/11 Memorial and Museum in New York for contributing this remnant of the North Tower as well as to Chancellor Merkel and the German people for donating this portion of the Berlin wall.

“It is truly fitting that these two artefacts now reside here so close together at the new Nato headquarters.

“And I never asked once what the new Nato headquarters cost – I refuse to do that.”