World leaders Boris Johnson, Justin Trudeau and Emmanuel Macron have been caught on video at a NATO reception, discussing Donald Trump in unflattering terms.

The pool footage – shot from across the room at Buckingham Palace – catches the British and Canadian PMs and the French president apparently discussing Mr Trump, although he is not mentioned by name.

“Is that why you were late?” Mr Johnson asks Mr Macron.

“He was late because he takes a 40-minute press conference off the top …” Mr Trudeau responds.

At that point, Mr Macron contributes something that isn’t picked up on the footage.

“You just watched his team’s jaws drop to the floor,” Mr Trudeau says.

Also involved in the discussion is Princess Anne, the Queen’s only daughter. Nothing she says is caught on the footage.

.@JustinTrudeau, @EmmanuelMacron, @BorisJohnson and other VIPs shared a few words at a Buckingham Palace reception Tuesday. No one mentions @realDonaldTrump by name, but they seem to be discussing his lengthy impromptu press conferences from earlier in the day. (Video: Host Pool) pic.twitter.com/dVgj48rpOP

— Power & Politics (@PnPCBC) December 3, 2019

Earlier, the US President and Mr Macron had clashed over the future of the NATO alliance and how to deal with the nearly 10,000 ISIS prisoners held in Syria at a media conference in London.

Mr Trump accused Mr Macron of “insulting” NATO allies, after the French president’s claim that the alliance was “brain dead”.

Mr Macron’s statement followed Turkey’s incursion against the Kurds in northern Syria without warning other NATO members – a move that alarmed other allies.

At a breakfast meeting with NATO secretary-general Jens Stoltenberg, Mr Trump said Mr Macron had been “very insulting to a lot of different forces”.

“It is a very, very nasty statement. I think they have a very high unemployment rate in France. France is not doing well economically at all.”

Mr Trudeau had also sat beside Mr Trump as he made baffling statements on a range of issues ahead of the two-day summit.

Topics included climate change, North Korea, deadly protests in Iran and the looming British election.

“I believe very strongly in very, very crystal clear, clean water and clean air. That’s a big part of climate change,” he said.

“I also see what’s happening with our oceans, where certain countries are dumping unlimited loads of things in it. They float. They tend to float toward the United States. I see that happening, and nobody’s ever seen anything like it, and it’s gotten worse.”

On North Korea, he said he had confidence in leader Kim Jong-un but noted he “likes sending rockets up, doesn’t he?”

“That’s why I call him Rocket Man,” Mr Trump said.

On the deadly disturbances in Iran that have become the bloodiest anti-government protests since the 1979 Islamic revolution, he said: “Iran is killing perhaps thousands and thousands of people right now as we speak. That is why they cut off the internet – so people can’t see what is going on.

“Not just small numbers, which are bad, big numbers which are really bad, and really big numbers.”

Asked about Britain’s looming December 12 election, the President said he did not want to complicate the vote. However, he still appeared to endorse Mr Johnson: “I think Boris is very capable and I think he’ll do a good job,” he said.

And British opposition leader Jeremy Corbyn?

“I can work with anybody. I’m a very easy person to work with,” Mr Trump said.

-with AAP