BRUSSELS - US President Donald Trump traded barbs with German Chancellor Angela Merkel at a tense Nato summit Wednesday after he accused Berlin of being “captive” to Russia and demanded it immediately step up defence spending.

President Trump shocked allies at the summit by suddenly demanding that members double their defence spending commitments. Trump’s surprise demand came after he clashed with Chancellor Angela Merkel.

The summit in Brussels is shaping up as the alliance’s most difficult in years, against a backdrop of deepening transatlantic tensions in fields ranging from trade to energy and defence.

Nato allies agreed at their Wales summit in 2014 to try to spend two percent of GDP on defence within 10 years, but the White House said Trump suggested that was not enough.

“During the president’s remarks today at the Nato summit he suggested that countries not only meet their commitment of two percent of their GDP on defence spending, but that they increase it to four percent,” White House spokeswoman Sarah Sanders said.

“The president raised this same issue when he was at Nato last year. President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations.”

Bulgarian President Rumen Radev confirmed that Trump had made the demand and asked what it meant for the future of the alliance that has been the bedrock of European security for 70 years. “Nato is not a stock exchange where you can buy security. Nato is an alliance of sovereign countries united by strategic targets and common values,” he told reporters.

All 29 NATO leaders including Trump backed a joint statement committing themselves to greater “burden sharing” and to the alliance’s founding commitment that an attack on one member is an attack on them all - with no mention of the four percent.

Trump arrived on the back of a barrage of criticism of Europe on issues ranging from trade to energy and above all his claims that the continent freeloads on the back of America for its defence.

He then set the tone for the day with a blistering attack on key ally Germany at a breakfast meeting with Nato chief Jens Stoltenberg.

“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 criticised.

“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.”

Merkel, who grew up in communist East Germany, shot back that she knew what it meant to be under Kremlin domination and Germany had the right to make its own policy choices.

“I myself have also experienced a part of Germany being controlled by the Soviet Union,” she said.

“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.”

The pair later met for a one-on-one meeting and while Trump insisted they had a “very very good relationship”, their frosty body language suggested otherwise.

Merkel said she welcomed the chance to have an “exchange of views” with Trump.

Trump has long complained that European Nato members do not pay enough for their own defence, accusing them of freeloading on America and singling out Germany for particular criticism.

“These countries have to step it up - not over a 10 year period, they have to step it up immediately,” Trump said. “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 defence more equally.

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

Nato officials and diplomats will try to promote an image of unity at the summit in the face of growing unease about the threat from Russia, but with the row between Merkel and Trump it may prove difficult to paper over the cracks.

The mercurial tycoon said before leaving Washington that his meeting in Helsinki with Russian President Vladimir 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.

Trump ramped up his rhetoric ahead of the talks, explicitly linking Nato with the transatlantic trade row by saying the EU shut out US business while expecting America to defend it.

EU President Donald Tusk stepped up to the fight with his own salvo against Trump on Tuesday, telling him to “appreciate your allies” and reminding him Washington that Europe had come to its aid following the 9/11 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.

Trump will meet the Russian leader in the Finnish capital on July 16 for their first summit amid an ongoing investigation in the US into possible Trump campaign collusion with Russia.

European diplomats fear a repeat of last month’s divisive G7 in Canada, when Trump clashed with his Western allies before meeting North Korean dictator Kim Jong Un at a summit and praising him as “very talented”.

There have been fears that Trump, keen to be seen to make a breakthrough with the Kremlin strongman, might make concessions that would weaken Western unity over issues such as Ukraine and Syria.

US ambassador to Nato Kay Bailey Hutchison urged allies to look beyond Trump’s rhetoric and focus on the summit declaration - which the US is expected to back - which will be the basis for the alliance’s work over the coming years.

And she said she expected Trump to recommit to one of the founding articles of Nato - Article 5 - which holds that an attack on one member is an attack on them all.