Donald Trump has ignited a fresh diplomatic row with French President Emmanuel Macron, accusing him of “insulting” Nato allies.

The US president, in London for a two-day meeting of alliance leaders, said Mr Macron had been “very disrespectful” after he claimed the alliance was “brain dead”.

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

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Speaking during a breakfast meeting with Nato secretary general Jens Stoltenberg at the US ambassador’s residence, Mr Trump said: “I think that is 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.

“It is a very tough statement to make when you have such difficulty in France, when you look at what is going on with the yellow vests.

“They have had a very rough year. You just can’t go around making statements like that about Nato. It is very disrespectful.”

Mr Trump’s outspoken intervention threatens to derail Boris Johnson’s hopes of acting as peacemaker at the meeting being held to mark the alliance’s 70th anniversary.

He was due to host a meeting with Mr Macron, Turkish President Recep Tayyip Erdogan and German Chancellor Angela Merkel in Downing Street on Tuesday ahead of the main gathering.

Mr Erdogan has already dismissed Mr Macron’s comments – accusing him of “a sick and shallow understanding” of terrorism, suggesting he was the one who was “brain dead”.

However there was widespread dismay in European capitals at the Turkish offensive last October against the Kurds – seen as crucial allies of the West in the fight against Islamic State (IS).

The incursion – which went ahead after Mr Erdogan was effectively given the green light by Mr Trump – was widely regarded as strengthening the position in Syria of Russia and of the Assad regime, while triggering another humanitarian crisis.

There was further alarm among alliance members when Mr Erdogan chose to to purchase Russian air defence systems – seen as a further weakening of his commitment to the alliance.

Ahead of the talks, Downing Street said that Mr Johnson would use the meeting to appeal for alliance unity in the face of continuing strains.

“The PM’s position is that Nato is the most enduring and successful alliance in military history and that it continues to adapt to the evolving threats that we face.

“It is the cornerstone of Euro-Atlantic security and it helps to keep a billion people safe.

“The PM will emphasise that all members must be united behind shared priorities so Nato can adapt to the challenges ahead.”

The message was underlined by Defence Secretary Ben Wallace, who said Nato’s “comparative advantage” over its competitors had always hinged on its unity.

Speaking at a Nato Engages conference ahead of the main gathering, he said they “must stand together – no side deals, no separate voices”.

“While differences of opinion are normal in any democratic organisation, we ultimately succeed because each of us trusts the other will have our back,” he said.

Mr Trump has previously berated the alliance’s European members for failing to contribute more financially to the costs of their collective defence.

However, watched by Mr Stoltenberg, he said the alliance had shown “great purpose” in adapting to evolving threats.

The Nato secretary general also dismissed Mr Macron’s criticism.

“Nato is active, Nato is agile, Nato is adapting. We have just implemented the largest reinforcement of collective defence since the Cold War,” he said.

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The formal Nato events will begin with a reception at Buckingham Palace hosted by the Queen on Tuesday evening.

The main talks will then take place on Wednesday at The Grove, a country house hotel near Watford.

The meeting is expected to consider new threats, including in the areas of cyber and space, after the alliance last month declared space one of its operational domains alongside air, land, sea and cyber.