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

The two-day meeting in Brussels was already shaping up to be the alliance's most difficult in years, with Europe and the United States engaged in a bitter trade spat and Trump demanding that NATO allies "reimburse" Washington for the cost of defending the continent.

European alliance members were braced for criticism from Trump on military 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 criticized.

"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."

In response, German Chancellor Angela Merkel said Germany had the right to make its own policy choices.

"I am very glad that we are united today in freedom as the Federal Republic of Germany and that we can therefore also make our own independent policies and make our own independent decisions," said the German leader who grew up in East Germany.

Trump was due to hold a one-on-one meeting with Merkel later on Wednesday and will meet Russian President Vladimir Putin in Helsinki, Finland, on Monday.

Trump has long complained that European NATO members do not pay enough for their own defense.

NATO allies agreed in 2014 to move toward spending 2 percent of GDP on defense by 2024. Germany, Europe's biggest economy, now spends 1.24 percent of GDP on defense, compared with 3.5 percent for the US.

'Direct language'

"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 defense more equally.

Canadian Prime Minister Justin Trudeau, whose country also lags on the 2 percent pledge, said the focus should be on "outputs" rather than on how much is spent.

EU President Donald Tusk stepped up to the fight with his own salvo against Trump.

"Dear America, appreciate your allies, after all you don't have that many," Tusk said on Tuesday, before reminding Trump that European troops helped the US after the Sept 11, 2001 attacks.

"Please remember this tomorrow when we meet at the NATO summit, but above all when you meet President Putin in Helsinki. It is always worth knowing who is your strategic friend and who is your strategic problem," he said.

During his campaign for president, Trump called NATO "obsolete" and suggested the US might not come to the defense of members if they found themselves under attack - a shift that would represent a fundamental realignment of the modern world order.

Trump had said his meeting in Helsinki with 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.

AFP - Reuters - AP