US President Donald Trump called today for Nato leaders to double their defence spending commitments, hours after he delivered a blistering tirade against Germany and other allies.

Although Trump joined fellow Nato leaders in approving a sweeping set of plans to bolster defences against Russia and terrorism, the US President has complained that Europe has been taking advantage of US military support for the continent, and he urged his counterparts in a private session to substantially raise a defense spending goal on which many are already falling short.

Not even the US - which spends more money on defence than any other nation in the world - meets Trump's new goal of annual spending of 4 per cent of nations' gross domestic product. Washington spent 3.6 per cent last year.

"During the President's remarks today at the Nato summit he suggested that countries not only meet their commitment of 2 per cent of their GDP on defence spending, but that they increase it to 4 per cent. The President raised this same issue when he was at Nato last year," White House spokeswoman Sarah Huckabee Sanders said.


"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations," she said.

Asked at a news conference about Trump's demands on defence spending, Nato Secretary-General Jens Stoltenberg suggested that the focus should be on getting every member country to reach the current goal of 2 per cent. Only eight of 29 Nato countries are on track to meet the 2 per cent goal this year.

Despite Trump's pugnacious posture and rhetoric, allies sought to project unity at the conclusion of meetings in Brussels.

"We do have disagreements, but most importantly, we have decisions that are pushing this alliance forward and making us stronger," Stoltenberg said. "At the end of the day, we all agree that North America and Europe are safer together."

Trump raised the spending issue during his remarks in the first and main session of the Nato summit.

The decision to sign on to the Nato defence plans plans suggested that Trump is holding back from slashing support for the alliance, despite his anger over what he says is Europe's taking advantage of the US security umbrella. Nato leaders are still concerned that he will make concessions to Russian President Vladimir Putin when the two meet on Monday in Helsinki.

As the day began, Trump wasted no time going on the offensive as he began a week of high-stakes diplomacy on both sides of the former Cold War divide.

The series of meetings - beginning with Nato and capped by a summit with Putin - has been largely framed around Trump's claims that Washington bears an unfair burden to help protect its allies.

"Germany, as far as I'm concerned, is captive to Russia because it's getting so much of its energy from Russia," Trump told Nato Secretary-General Jens Stoltenberg in a fiery on-camera exchange that was among the harshest in the history of the post-World War II alliance.

"We have to talk about the billions and billions of dollars that's being paid to the country we're supposed to be protecting you against," Trump said, referring to European purchases of Russian natural gas.

Trump has complained bitterly about Europe's lagging defense spending, saying that Nato nations were taking advantage of US military largesse at the same time they were offering unfair trade terms to US businesses.

A favourite target of his ire has been Germany, which has not met its Nato spending commitments and is beginning construction on a second natural gas pipeline to Russia.

Germany and other European Nato partners argue, however, that they have boosted contributions to the military alliance and plan to kick in even more in coming years. Germany's leadership has said the pipeline is a private business decision and they have been reluctant to interfere.

The accusation of Russian influence may have been particularly biting to German Chancellor Angela Merkel, who grew up in Communist-controlled East Germany.

"I myself experienced that a part of Germany that was controlled by the Soviet Union, and I am very happy today that we are united in freedom as the Federal Republic of Germany," Merkel told reporters as she entered Nato. "We decide our own policies and make our own decisions."

The US leader travelled to Europe saying that a summit with Putin will be the easiest of his week of diplomacy - an unusual assertion that upended Nato leaders' belief that the alliance should project a strong and united front against a strategic rival.

Trump has preferred to take aim at allies.

Even Stoltenberg - a mild-mannered former Norwegian Prime Minister who has cultivated a positive relationship with Trump - appeared reduced to spluttering as Trump cut him off after he started to explain that allies traded with Russia even during the Cold War. Earlier in the exchange, Trump demanded credit from Stoltenberg for forcing an increase of Nato defence budgets.

"It was also because of your leadership," Stoltenberg told Trump. Budget increases started after Russia's 2014 annexation of Ukraine's Crimean Peninsula, and they have accelerated in the Trump era in response to the US President's criticism.

"We're supposed to protect Germany but they're getting their energy from Russia," Trump told Stoltenberg, as aides on both the US and Nato side of a long table shifted in their seats and sat stonefaced. Chief of Staff John Kelly jerked his head away as US Ambassador to Nato Kay Bailey Hutchison looked up at the ceiling. "So explain that," Trump said. "And it can't be explained, and you know that."

Trump's criticism set off immediate anxiety in Germany. Munich's Süddeutsche Zeitung headlined its story: "It is not only bad, it is catastrophic."

Germany's energy relationship with Russia has long frustrated Washington and Eastern Europe, who fear that the Nord Stream 2 pipeline that bypasses the Baltic nations and Poland could be used to cut them off from crucial energy supplies. Former German Chancellor Gerhard Schroeder is a top executive at the Russian-Government-controlled company that runs the Nord Stream 2 pipeline.

Trump has promoted exports of U.S. natural gas to Europe as an alternative to Russia as a supply source, although U.S. gas is far more costly because of the expense of shipping it.

Trump's laser-focus on Germany has unsettled Berlin, which had grown accustomed to a strong relationship with President Barack Obama. Trump plans to meet one-on-one with Merkel, when he will reiterate the same tough message to her face, White House spokeswoman Sarah Huckabee Sanders said.

Journalists were allowed briefly into the room at the end of their meeting, but both leaders avoided publicly blasting each other face-to-face.