US President Donald Trump has accused Germany of being a “captive” of Russia because of its energy reliance in a startling outburst before a NATO summit where he pressed allies to more than double spending on their own defence.

Having lambasted NATO members for letting US taxpayers pay to keep them safe from Russia by failing to spend a target two per cent of national income on defence, Mr Trump told fellow leaders he would prefer a target of four per cent of GDP, similar to US levels, according to US and foreign officials.

That would represent a massive upheaval of budgetary priorities in Europe where Germany and many others have pledged only to reach two per cent by 2030.

Asked to confirm Mr Trump’s proposal, NATO Secretary-General Jens Stoltenberg did not give a direct answer but said the aim first was to reach two per cent.

A White House official said four per cent was not a formal proposal. US defence spending in 2017 was 3.6 per cent of GDP.

Leaders formally reiterated their “unwavering commitment” to raising spending and to better share the costs of joint defence.

Before the other leaders gathered, Mr Trump told Mr Stoltenberg that Germany was wrong to support an $US11 billion Baltic Sea pipeline to import even more Russian gas while being slow to meet targets for NATO spending to protect against Russia.

“We’re supposed to be guarding against Russia and Germany goes out and pays billions and billions of dollars a year to Russia,” Mr Trump said in the presence of reporters on Wednesday.

Mr Trump appeared to substantially overstate German reliance on Russian energy and to imply Berlin was funding a pipeline which German Chancellor Angela Merkel says is a commercial venture.

Mr Trump and Ms Merkel later held businesslike talks on the sidelines. Mr Trump said he had a “very, very good relationship” with Ms Merkel, who described the two as “good partners”.

Donald Trump said Germany was wrong to support an $US11 billion Baltic Sea pipeline. Photo: Getty

A source close to French President Emmanuel Macron said Mr Trump had voiced his “personal attachment” to Europe and gave “rather positive and constructive messages” to his allies.

But with tensions in the Western alliance smouldering over Mr Trump’s trade tariffs on European steel and his demands for more contributions to ease the burden on US taxpayers, his earlier remarks fuelled concerns among allies for the US role in keeping the peace that has reigned since World War Two.

NATO’s Stoltenberg later told reporters that Mr Trump, who will meet Russian President Vladimir Putin on Monday, had used “very direct language” but that NATO spending was rising strongly.

After joking that his breakfast with Mr Trump at the US ambassador’s residence had been paid for by the United States, the NATO chief was frank about the impact of Trump’s criticism on the Western allies 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.”

-AAP