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US President Donald Trump has accused Germany of being a "captive" of Russia due to its energy reliance, before a NATO summit where he pressed allies to more than double defence spending.
Having lambasted NATO members for failing to reach a target of spending 2% of national income on defence, Trump told fellow leaders in Brussels on Wednesday he would prefer a goal of 4%, similar to US levels, officials said.
That would represent a massive upheaval of budgetary priorities in Europe where Germany and many others have pledged only to reach 2% by 2024 or later, and it was not clear what allies would spend the money on.
At the end of the first session of a two-day summit, NATO Secretary-General Jens Stoltenberg said the aim first was to reach 2%, but moments later Trump tweeted that allies were undercutting the United States on trade and needed to immediately up spending.
The exchange was part of an uncomfortable day as anxious Western allies were subjected to the US president's "America first" approach. His comment that Germany was controlled by Russia earned a rebuke from Berlin.
"We are not prisoners, neither of Russia nor of the United States," Germany's Foreign Minister Heiko Maas told reporters.
But Trump was also at times enthusiastic and affable, exchanging embraces and handshakes with fellow leaders, posing for photographs in front of a NATO military band and mingling at a summit dinner in a Brussels museum.
Unlike a fraught Group of Seven summit in Canada in June, Trump did not block a final leaders' statement, which paved the way for Macedonia to start NATO membership talks and a modernisation of the alliance to better deter Moscow.
Earlier, Trump strode into NATO's new glass-and-steel headquarters and into the amphitheatre-like North Atlantic Council decision-making chamber to shake hands with leaders - but only after publicly railing against Germany, one of NATO's top European powers, at a breakfast meeting with Stoltenberg.
Trump told Stoltenberg Germany was wrong to support a $US11 billion ($NZ16.2 billion) Baltic Sea pipeline to import even more Russian gas while being slow to meet targets for NATO spending.
"We're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia," Trump said in the presence of reporters.
"We're protecting Germany, we're protecting France, we're protecting all of these countries. And then numerous of the countries go out and make a pipeline deal with Russia where they're paying billions of dollars into the coffers of Russia," he said. "I think that's very inappropriate."
Trump appeared to substantially overstate German reliance on Russian energy and to imply Berlin was funding a pipeline that Chancellor Angela Merkel says is a commercial venture.
She hit back by contrasting her own experience of growing up in Soviet-controlled East Germany with the sovereign, united Germany now playing a major role in NATO.
Trump and Merkel later held businesslike talks on the sidelines. Trump said he had a "very, very good relationship" with Merkel who described the two as "good partners".
A source close to French President Emmanuel Macron said Trump had voiced his "personal attachment" to Europe and gave "rather positive and constructive messages" to his allies.
"There is no break-up between America and Europe," the source said after Macron and Trump held "friendly" talks.
With tensions in the alliance smouldering over Trump's trade tariffs on European steel, his earlier comments fuelled concerns about the US role in keeping the peace that has reigned since World War 2.
Belgian Prime Minister Charles Michel called for "fair play" and more respect for multilateralism.
Baltic leaders, fearful of any repeat of Russia's annexation of Crimea, called for unity, while Slovak President Andre Kiska said his country was "one of the good guys" because it was increasing defence spending.
Those comments underscored the risks to Trump's strategy by dividing allies between those who spend more on defence and those who do not, such as Belgium, Spain, Italy and Luxembourg, but who contribute with troops to NATO missions.
NATO's Stoltenberg later told reporters that Trump, who will meet Russian President Vladimir Putin on Monday, had used "very direct language" but that NATO spending was rising strongly.
The NATO chief was also frank about the impact of Trump's stance at a broader level.
"There are disagreements on trade. This is serious. My task is to try to minimise the negative impact on NATO," he said.
"So far it hasn't impacted on NATO that much. I cannot guarantee that that will not be the case in the future. The transatlantic bond is not one, there are many ties, some of them have been weakened."