US President Donald Trump has repeatedly accused Washington's allies of failing to pay their fair share, straining ties in the alliance as it celebrates its 70th birthday.

Officials are worried that Trump's frustration -- combined with doubts about the direction of the alliance expressed by France's President Emmanuel Macron -- will overshadow next week's summit.

So NATO has coordinated a week of spending announcements in Brussels to encourage a display of unity in London.

On Friday, Stoltenberg said European countries and Canada would increase defence spending by 4.6 percent in 2019, stressing that they would have spent a combined $130 billion between 2016 and the end of next year.

This would keep the countries on course to spend two percent of their GDP on defence by 2024. But only nine countries have hit the target so far, and Germany admits it will not do so.

"President Trump is right about the importance of the European allies and Canada spending more and he has conveyed that message very clearly to allies several times," Stoltenberg told reporters.

"But 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."

- Bid to mollify Trump -

Stoltenberg said the NATO members apart from the US had mainly been cutting defence budgets before 2014, when they signed up to a pledge to "move towards" the two percent of GDP target by 2024.

But Chancellor Angela Merkel admitted this week that Germany, Europe's economic powerhouse and a frequent target of Trump's ire, will not hit the goal until "the early 2030s".

France's Macron outraged the allies this month by demanding a new strategy that focused more on terrorism and less on deterring Russia, complaining that NATO was experiencing "brain death".

Stoltenberg, meanwhile, is attempting to mollify Trump ahead of the summit by talking up a billion-dollar contact with US planemaker Boeing to upgrade NATO reconnaissance planes.

And the allies have agreed to lower the cap on US support for NATO's relatively small $2.5-billion operating budget, meaning Germany and other Europeans -- but not France -- pay a bigger share.

The 29 NATO leaders will gather in Watford near London on Tuesday and Wednesday next week.

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