Donald Trump has lashed out at Emmanuel Macron on the first morning of a two-day Nato summit in London.

At a news conference held to celebrate the success of the alliance, with the Nato secretary general, Jens Stoltenberg, by his side, the US president focused his ire on his French counterpart.

After Macron had tried to shake up the agenda for the London summit by calling the 70-year western alliance “brain dead”, Trump said he could see Paris “breaking away” from Nato.

“Nato serves a great purpose,” Trump said. “I think that’s very insulting,” he said of Macron’s comment, branding it a “very, very nasty statement essentially to 28 countries”.

He added: “Nobody needs Nato more than France. It’s a very dangerous statement for them to make.”

Asked whether the US alliance with Nato was shaky, Trump denied it, but said: “I do see France breaking off … I see him breaking off.”

Trump defended Stoltenberg, boasting that Nato members had massively increased their defence spending thanks to his pressure – but then reiterated his complaints about European spending.

“When I came in, I was angry at Nato, and now I’ve raised $130bn,” Trump said, referring to the sum Stoltenberg says Canada and European members will have added to defence budgets by next year.

“And yet you still have many delinquent – you know, I call them delinquent when they’re not paid up in full,” he said.

Only nine of Nato’s 29 members spend 2% of their GDP on defence. Trump cited Germany as falling short, spending only 1% to 1.3%.

Leaders of the 29 allies are descending on London to lock horns over spending and how to deal with Russia in a test of unity as Nato seeks to assert its relevance.