Under fire for his warm embrace of Russia's Vladimir Putin, US President Donald Trump has jolted the NATO summit by turning a spotlight on Germany's ties to Russia and openly questioning the value of the military alliance that has defined American foreign policy for decades.

Key points: Trump says Germany is "captive" to Russia

Trump says Germany is "captive" to Russia Merkel says Trump has no standing to judge Germany

Merkel says Trump has no standing to judge Germany US President also ramps up pressure on NATO chief

Mr Trump declared that a joint natural gas pipeline venture with Moscow has left Angela Merkel's Government "totally controlled" and "captive to Russia".

In a stroke he had shifted attention away from his own ties to the Kremlin just days before he meets one-on-one with Mr Putin.

The President questioned the necessity of the alliance that formed a bulwark against Soviet aggression, tweeting after a day of contentious meetings: "What good is NATO if Germany is paying Russia billions of dollars for gas and energy?"

Sorry, this video has expired Donald Trump claims Germany is 'totally controlled' by Russia

German Chancellor Merkel hit back immediately, not only denying Mr Trump's contention but suggesting that his comfortable upbringing in the US gave him no standing to spout off on the world stage about Germany.

Ms Merkel drew on her own background growing up in communist East Germany behind the Iron Curtain.

"I've experienced myself a part of Germany controlled by the Soviet Union, and I'm very happy today that we are united in freedom as the Federal Republic of Germany and can thus say that we can determine our own policies and make our own decisions and that's very good," she said.

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Mr Trump demanded via Twitter that members of the North Atlantic Treaty Organisation "must pay 2% of GDP IMMEDIATELY, not by 2025" for their military efforts.

He then rattled US allies further by privately suggesting member nations should spend 4 per cent of their gross domestic product on the military — more than even the United States currently pays, according to NATO statistics.

It was just the latest in Mr Trump's demands and insults that critics fear will undermine a decades-old alliance launched to counter-balance Soviet aggression after World War II.

It came just days before Mr Trump planned to sit down with Mr Putin in Finland at the conclusion of what has become a contentious European trip.

Mr Trump has spent weeks berating members of the alliance for failing to increase military spending, accusing Europe of freeloading off the US and even raising doubts about whether he would come to members' defence as required if they were ever attacked.

Donald Trump and Vladimir Putin are set to meet in Helsinki on July 16. ( Jorge Silva: AP )

Mr Trump's criticism accelerated during a pre-summit breakfast, when he traded his usual long-distance Twitter attacks for a face-to-face confrontation with NATO Secretary-General Jens Stoltenberg.

"We're supposed to protect you against Russia but they're paying billions of dollars to Russia and I think that's very inappropriate," Mr Trump said, repeatedly describing Germany as "captive to Russia" because of the energy deal.

Hours after the breakfast, Ms Merkel and Mr Trump appeared to play nice as they met along the summit's sidelines.

"We're having a great meeting. We're discussing military expenditure, talking about trade," Mr Trump said.

"We have a very, very good relationship with the Chancellor. We have a tremendous relationship with Germany."

'I think the Secretary-General likes Trump'

Donald Trump (right) speaks with NATO secretary-general Jens Stoltenberg before their bilateral breakfast. ( AP: Pablo Martinez Monsivais )

Mr Trump is expected to continue hammering jittery NATO allies about their military spending during the summit, which comes amid increasingly frayed relations between the "America first" President and the United States' closest traditional allies.

Mr Stoltenberg, for his part, has credited Mr Trump for spurring NATO nations to spend more on defence, noting the Europeans and Canada are projected to spend around $US266 billion ($359 billion) more by 2024.

He said last year's increases marked the largest in a generation.

Arriving for his meeting with Mr Stoltenberg, Mr Trump told the NATO chief that "because of me they've raised about $40 billion over the last year".

"So I think the Secretary-General likes Trump. He may be the only one, but that's OK with me," he said.

Brussels is Mr Trump's first stop of a week-long European tour that will include stops in London and Scotland, as well as a highly-anticipated meet with Russia's Vladimir Putin.

Sorry, this video has expired Former policy chief at NATO Fabrice Pothier discusses the NATO summit

AP/Reuters