World Bulletin / News Desk

US President Donald Trump launched a blistering attack on Germany at the start of a tense NATO summit Wednesday, accusing Berlin of being "captive" to Russia and demanding it and other allies immediately step up defence spending.

European alliance members were braced for criticism from Trump on defence spending, but his furious tirade at what should have been an amicable breakfast meeting appeared to take even NATO chief Jens Stoltenberg by surprise.

"Germany is a captive of Russia because it is getting so much of its energy from Russia," Trump said, taking particular aim at the proposed Nord Stream II gas pipeline, which he has previously criticised.

"Everybody's talking about it all over the world, they're saying we're paying you billions of dollars to protect you but you're paying billions of dollars to Russia."

The US president will hold a one-on-one meeting with German Chancellor Angela Merkel at the NATO summit on Wednesday, the White House said.

Trump has long complained that European NATO members do not pay enough for their own defence, accusing them of freeloading on America and singling out Germany for particular criticism.

NATO allies agreed at the Wales summit in 2014 to move towards spending two percent of GDP on defence by 2024. But Germany, Europe's biggest economy, spends just 1.24 percent of GDP on defence, compared with 3.5 percent for the US.

"These countries have to step it up -- not over a 10 year period, they have to step it up immediately," Trump said.

"We're protecting Germany, France and everybody... this has been going on for decades," Trump said. "We're not going to put up with it, we can't put up with it and it's inappropriate."

Stoltenberg acknowledged that Trump had expressed himself in "very direct language" but insisted that away from the fiery rhetoric the allies all agree on fundamental issues: the need to boost NATO's resilience, fight terror and share the cost of defence more equally.

Canadian Prime Minister Justin Trudeau, whose country also lags on the two percent pledge, said the focus should be on "outputs" rather than on how much is spent.

"You can try to be a bean-counter and look at exactly how much of this, and how much that, but the fundamental question is: is what you are doing actually making a difference?" Trudeau said.

NATO officials and diplomats will try to promote an image of unity at the summit in the face of growing unease about the threat from Russia, but after Trump's attack it may prove difficult to paper over the cracks.

The mercurial tycoon said before leaving Washington that his meeting in Helsinki with Russian President Vladimir Putin on Monday "may be the easiest" part of his European tour, which also includes a trip to Britain, where the government is in crisis over Brexit.