Macron had tried to shake up the agenda for the London summit by branding the 70-year-old Western alliance "brain dead", but Trump slapped him down and warned that he could see Paris "breaking away" from NATO.

"NATO serves a great purpose," Trump said, at a joint press appearance with alliance Secretary General Jens Stoltenberg.

"I think that's very insulting," he said of Macron's comment, branding it a "very, very nasty statement essentially to 28 countries.

"Nobody needs NATO more than France," he said. "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 have 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 130 billion dollars," 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 two percent of their GDP on defence.

Trump cited in particular Germany as falling short, spending only one to 1.3 percent.

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

'Brain death'

If the Macron comments set an angry tone for the meeting, there are also expected to be clashes with Turkey's President Recep Tayyip Erdogan, who was also furious with the French leader.

"First of all, have your own brain death checked. These statements are suitable only to people like you who are in a state of brain death," he said last week.

French officials summoned the Turkish envoy in Paris to complain while a US administration official predicted that many members would tackle Turkey over its purchase of a Russian S-400 air defence system.

Turkey, in turn, has warned it will block a NATO plan to defend Baltic countries unless the alliance recognises a Kurdish militant group as terrorists, Erdogan said before the summit.

It was reported last week that Ankara was blocking NATO's new Baltic defence plan, demanding greater support in its fight against the Syrian Kurdish People's Protection Units (YPG).

NATO has mooted a plan to bolster the defences of Poland, Estonia, Lithuania and Latvia against a potential attack from Russia, though details remain unclear.

Macron and Erdogan will come face to face on Tuesday in a four-way meeting with Merkel and British Prime Minister Boris Johnson, whose spokesman said he would be emphasising the need for NATO unity.

"We can see this as likely the tale of three egos," said Amanda Sloat, a former senior diplomat and a fellow of the Brookings Institution, warning that Trump, Macron and Erdogan were the figures to watch.

The summit also comes at an unexpectedly awkward moment for Johnson, with Britain in the midst of a frenetic election campaign.

Normally a summit like this would give the PM of the day a boost but with Trump deeply unpopular among many British voters, his visit is a potential banana skin for Johnson.

Where next?

The substance of the meeting is thin, with only one three-hour session planned, where leaders are expected to sign off on a set of decisions already taken by NATO foreign and defence ministers.

These include making space a full domain of conflict -- alongside land, sea, air and cyber space -- as well as a new report on how the alliance should approach China and its growing international assertiveness.

What is likely to be more significant in the longer run is the fallout from Macron's broadside, in which he complained NATO talks too much about money and not enough about strategic priorities.

In response, Germany has suggested setting up an expert panel to look at how NATO can be adapted to address political questions more effectively. —AFP