"He's two-faced," Trump told reporters on the sidelines of the NATO summit, saying he had called out Trudeau on defence spending "and he's not very happy about it".

Trump was speaking before holding talks with German Chancellor Angela Merkel after the 29 leaders of NATO countries met in Watford, north of London.

"With Trudeau, he's a nice guy. I find him to be a nice guy, but the truth is, I called him out on the fact that he's not paying two percent and I guess he's not very happy about it," he said, referring to an alliance commitment to spend two percent of GDP on defence.

Trudeau was picked up by microphones at a Buckingham Palace reception for NATO leaders on Tuesday evening apparently laughing about an impromptu press conference Trump did earlier.

British Prime Minister Boris Johnson can be heard asking France's President Emmanuel Macron: "Is that why you were late?"

Canada's Prime Minister Justin Trudeau interjects: "He was late because he takes a 40 minute press conference off the top."

In the video, Macron appears to tell an anecdote about the encounter as Britain's Princess Anne and Dutch Prime Minister Mark Rutte look on, but the French leader has his back to camera and he is inaudible amid the hubbub.

"Oh, yeah, yeah, he announced..." an amused Trudeau declares, adding: "You just watched his team's jaw drop to the floor."