And at this week's NATO summit in London, their bemusement became embarrassingly public when video emerged of four of Trump's counterparts mocking his lengthy interventions.

Never one to take a slight lying down, Trump responded by reviving his feud with Canada's Prime Minister Justin Trudeau, branding him "two-faced" over his leading role in the exchange.

The prickly president's honour was apparently satisfied by this, as he was later overheard on a second hot mic boasting: "That was funny when I said that guy was two-faced."

And in a final irony, Trump cancelled his scheduled press conference at the end of the summit, concluding not unreasonably that he had already said all he had to say during his ad hoc Q&As.

His early departure might have come as a relief to summit organisers. NATO officials had been dreading any Trump outburst that could derail their efforts to present a unified allied front.

And Britain's Prime Minister Boris Johnson will be glad that Trump did not embarrass him too greatly during an election campaign in which he has come under fire for his ties to the US leader.

It could have gone very differently, and NATO diplomats admitted that they feared the worst on Wednesday morning when video footage emerged of the leaders laughing about Trump at a Buckingham Palace reception.

The footage was shot by the British host's camera pool on Tuesday evening and spotted and subtitled by Canadian broadcaster CBC.

- Jaw drop -

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

Trudeau interjects: "He was late because he takes a 40 minute press conference off the top."

Macron's one-on-one pre-summit meeting with Trump had been proceeded by a lengthy question and answer session with the media, the first of many that took up several hours over the two days.

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's back is 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."

As he did at last year's NATO meeting, Trump threw out normal summit protocol and used his one-on-one appearances with allied leaders to field dozens of questions from the world's media.

He has condemned as "nasty" Macron's criticism of NATO as brain dead, branded European countries that have failed to meet military spending targets "delinquent" and railed against moves in Washington to impeach him.

After the summit, a flustered Johnson insisted that reports of the exchange were "nonsense", but Trump had clearly seen it and he was angry, particularly with the apparent ring-leader, Trudeau.

- Golf resort -

"He's two-faced," Trump told reporters, in yet another press appearance, this time before his head-to-head with German Chancellor Angela Merkel.

"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.

NATO's 29 members have a target of spending two percent of their GDP on defence, and Canada is among the majority in failing to do so.

Diplomats had been worried Trump would be angered by the exchange but in the end it was Trudeau who appeared chastened.

He told reporters he had merely been welcoming the US leader's abrupt announcement that next year's G7 summit would be held at his Camp David country retreat, and not at one of Trump's Florida golf resorts.

Trump had been due to give another news conference, this time on his own, after the leaders held a full three-hour closed-door summit session and issued a statement to celebrate their unity.

But, despite having been only too happy to talk to the press for the two days of the meetings, he abruptly cancelled this scheduled appearance.

The irony was not lost on Trump. "And then you know what they'll say?" he asked, in the accidentally recorded aside "'He didn't do a press conference! He didn't do a press conference!'"