National leaders will descend on London on Tuesday bracing for a scrap over spending and how to deal with Russia, in a huge test of unity within NATO -- billed by its own officials as the "most successful alliance in history".

US President Donald Trump has repeatedly accused European countries of failing to pay their way and will be looking for evidence they are stepping up defence spending.

France's Emmanuel Macron has despaired of the club's strategic direction, saying it is suffering "brain death" -- riling other leaders and drawing a rare public rebuke from German Chancellor Angela Merkel.

And on Friday Turkey's President Recep Tayyip Erdogan, furious at Western criticism of his operation in northern Syria against the Kurds, hit back at Macron in personal terms.

"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," Erdogan declared Friday.

In a televised speech, Erdogan said he would "say this at NATO".

French officials summoned the Turkish envoy in Paris to complain.

This combustible line-up is dropping into a Britain gripped by a frenetic national election campaign, with Prime Minister Boris Johnson's friendship with Trump under attack from opposition parties.

Personal duels aside, the NATO summit agenda is pretty thin. Secretary general Jens Stoltenberg is hoping simply to get the leaders to sign off on decisions already taken.

Last year's NATO summit in Brussels went off the rails when Trump launched a tirade at Merkel during a televised breakfast meeting.

It ended with him heading for a controversial one-on-one meeting with Russian President Vladimir Putin in Helsinki.

The week before this summit has seen a stage-managed series of spending announcements, all designed to send what one diplomat called a "political signal" to appease Trump.

- 'Trump is right' -

Stoltenberg was at pains to point out on Friday that Canadian and European defence spending has grown for four straight years and is on course to hit $130 billion next year.

He said Trump was right about Europe and Canada needing to spend more, but added that "European allies and Canada should not invest in defence to please President Trump".

"They should invest in defence because we are facing new challenges, our security environment has become more dangerous," he told reporters.

Stoltenberg is attempting to mollify Trump ahead of the summit by talking up a billion-dollar contract with US planemaker Boeing to upgrade the organisation's reconnaissance planes.

NATO members have also agreed to lower the cap on US contributions to the alliance's relatively small $2.5 billion operating budget, meaning Germany and other European countries -- but not France -- will pay more.

But such measures are a drop in the ocean compared to the tens of billions of dollars Europeans would have to spend to meet their promise to spend two percent of their GDPs on defence.

In 2014, the allies promised to meet this goal within a decade. But this week Merkel admitted that economic powerhouse Germany would not hit this sum before "the early 2030s".

Stoltenberg insists Trump's tone towards NATO has been more positive of late, but Macron's broadside an Economist interview earlier this month took many by surprise.

The French leader stood by his remarks after talks with Stoltenberg, saying NATO was failing to address relations with Russia and what do to about Turkey.

Macron's forthright comments have drawn sharp public criticism, both from Germany and from eastern European NATO countries that feel threatened by Russia.

Tomas Valasek, a former Slovak ambassador to NATO, said even if there was merit in opening debate, Macron had overstepped the mark.

"NATO leaders have a responsibility that thinktankers don't," said Valasek, now a senior fellow at the Carnegie Europe thinktank.

"If you run one of the nuclear powers and in some ways the most powerful military in Europe you don't want to feed the perception of NATO disunity and I'm afraid that's what he's done."

- 'No sympathy' -

However, some acknowledge that the time may be ripe for reflection.

"The reactions on the interview were very negative in the alliance -- there is no sympathy," one NATO diplomat said.

"At the same time, he triggered a very lively discussion on the question where the alliance stands and in which direction it will proceed."

At the London summit, leaders will consider separate French and German proposals for expert committees to mull how NATO can improve its strategic thinking.

Stoltenberg last week welcomed the German plan to create a group of experts -- chaired by Stoltenberg himself -- but was cool on the French plan.

No formal statement by all 29 leaders will be issued. Instead there will be a "short declaration on the 'success story of NATO'", a diplomat said.

pdw-dc/rmb/jxb

BOEING