US President Donald Trump launched a two-day NATO meeting Tuesday with a blistering attack on France's criticism of the alliance and on ‘delinquent’ members that do not pay their way.

At a news conference held to celebrate NATO's success in cajoling European allies to boost their defence spending, Trump lashed out at French President Emmanuel Macron.

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 he could see France ‘breaking away’ from NATO.

‘NATO serves a great purpose,’ Trump told reporters alongside 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.’

Stoltenberg renewed his own criticism of the French leader's remarks, saying allies ‘should never question the unity and the political willingness to stand together and to defend each other’.

Trump defended Stoltenberg, boasting that NATO members have massively increased their defence spending thanks to his pressure.

But the president then reiterated his complaints about European spending.

‘When I came in, I was angry at NATO, and now I've raised $130 billion,’ 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 1.2 percent.

Leaders of the 29 allies are descending on London to lock horns over spending and strategic direction 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 confront Turkey over its purchase of a Russian S-400 air defence system.

Erdogan, in turn, has warned that Turkey will block an update to NATO's plan to defend Baltic countries unless the alliance recognises the Kurdish militant YPG group as terrorists.

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.

Polish President Andrzej Duda played down the dispute, saying he believed ‘a good solution’ would be found.

But he weighed in on Macron, urging him to stop carping about NATO and come up with ‘concrete propositions’ for how to improve the alliance.

Russian President Vladimir Putin said Moscow was open to cooperation with NATO against joint threats such as terrorism.

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.

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.

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