US President Donald Trump denounced Justin Trudeau as "two-faced" on Wednesday after the Canadian prime minister was caught on camera laughing over Trump's press appearances during a chat with other leaders at a Buckingham Palace reception.

The incident came at a reception on Tuesday evening, after Trump had criticised French President Emmanuel Macron and other leaders during long question-and-answer sessions with reporters on the eve of a NATO summit.

Macron, Trudeau, British Prime Minister Boris Johnson and Dutch Prime Minister Mark Rutte were filmed chatting at the reception, along with Queen Elizabeth's daughter Princess Anne.

"Is that why you were late?" Johnson can be heard asking Macron.

"...It was like a 40-minute press conference," Trudeau said. "Yeah, yeah, yeah! Forty minutes."

Other words were exchanged but could not be heard, before Trudeau added with a chuckle: "I just watched his team's jaws drop to the floor."

When asked about Trudeau's comments on Wednesday, Trump replied: "He's two faced."