US President Donald Trump upended the NATO summit tonight by calling an emergency meeting of leaders and threatening that if all member countries do not immediately increase their defence spending commitments, the United States would go it alone, according to diplomats with knowledge of the private discussions.

It was not clear whether Trump was threatening a US withdrawal from NATO, but some diplomats perceived his comments that way.

Trump told NATO leaders that if they did not meet their defence spending targets of 2 percent of gross domestic product by January, the United States would go it alone, according to two officials briefed on the meeting. The officials said Trump threatened to "do his own thing."

Trump scolded leaders here and singled out some specific countries, including Germany and Spain, for failing to contribute more to their defences and for relying too heavily on the largess of the United States.

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Trump's demand jolted the transatlantic alliance at the conclusion of this week's Brussels summit and, in the words of one diplomat briefed on the events, sent "everyone into a tailspin." A morning of meetings to discuss the situation in Afghanistan, Georgia and Ukraine was scrambled to address Trump's spending concerns.

Throughout his presidency, Trump has railed against European allies for not paying more for the defences and becoming too dependent on the United States. And he continued beating that drum in a pair of tweets he fired off Thursday morning from the Brussels residence where he stayed, before arriving at NATO headquarters for meetings.

"Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more toward their protection from Russia," Trump wrote. "They pay only a fraction of their cost. The US pays tens of Billions of Dollars too much to subsidize Europe, and loses Big on Trade!"

Trump wrote in a second tweet, continuing a line of attack he opened Wednesday against Germany over its reliance on a gas pipeline from Russia: "On top of it all, Germany just started paying Russia, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russia. Not acceptable! All NATO Nations must meet their 2% commitment, and that must ultimately go to 4%!"

At a closed-door session Wednesday with NATO leaders, Trump called on member nations to increase their defence spending targets from 2 percent of each country's gross domestic product to 4 percent — a figure higher than what even the United States channels toward its military.

"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations," White House press secretary Sarah Huckabee Sanders said in a statement.

Trump began his trip here by scolding allies over breakfast Wednesday with NATO Secretary General Jens Stoltenberg.

"Many countries are not paying what they should," Trump said, seated across from a visibly rattled Stoltenberg. "And, frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them."