US President Donald Trump has launched a fresh attack on his NATO allies' failure to raise defence spending, prompting leaders to huddle in a special session excluding other summit participants.
At one point, in a break with protocol, a source said Trump addressed German Chancellor Merkel by her first name on Thursday and told her: "Angela, you need to do something about this."
Invited leaders from non-NATO countries Afghanistan and Georgia were asked to leave along with most NATO leaders' retinues of officials, as the heads of state and government of the Western alliance sought to deal with the man whose nation commands much of the budget and forces for Europe's defence.
Trump had opened the first day in Brussels on Wednesday with a public diatribe against Germany before the mood appeared to calm as the summit went into day two, focusing on operations beyond Europe.
But sources said Trump reopened in strong terms his demand that other countries greatly speed up a NATO target of spending at least two per cent of their GDP on defence, which now has a deadline of 2024 with get-out terms that can stretch it to 2030.
"The language was much tougher today," one said.
"His harshest words were directed at Germany, including by calling her Angela - 'You, Angela.'"
As well as Merkel, Spanish Prime Minister Pedro Sanchez and Charles Michel, the prime minister of Belgium, were also singled out by Trump for undershooting on their spending targets when US taxpayers, funding a defence budget worth about 3.6 per cent of their national income, foot much of NATO's bills.
Breaking from a carefully scripted session that was to focus on Ukraine and Georgia, one source said Trump "forcibly restated his position on wanting NATO members to reach 2 per cent spending target to a short a deadline".
Two NATO sources said, however, Trump had not issued a threat to pull the United States out of the alliance that it helped found to keep the peace in Europe after World War Two.
Leaders had gone into the second day, set to focus mainly on ending the long war in Afghanistan, with low expectations, showing the extent of European frustration with Trump's "America first" policies among the other 28 members of the alliance.
Trump took to Twitter to say publicly what he told NATO leaders privately on Wednesday, calling on all allies to meet a commitment agreed in 2014 to spend 2 per cent of economic output on defence to counter threats ranging from Russia's military modernisation to militant attacks on European cities.
He also renewed a demand that they even double that - though many question what they could usefully spend it on.
"All NATO Nations must meet their 2% commitment, and that must ultimately go to 4%!" Trump tweeted an hour before the second day of the summit got under way.