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 USA taxpayers, funding a defence budget worth about 3.6 percent of their national income, foot much of NATO's bills.
"Not acceptable!" he railed before arriving late at North Atlantic Treaty Organisation headquarters for morning meetings that will include talks with the leaders of Azerbaijan, Romania, Ukraine and Georgia. He said leaders committed to go to their parliaments to obtain the spending increases.
The U.S. Embassy has warned Americans to be careful in London during Trump's visit, recommending that they "exercise caution if unexpectedly in the vicinity of large gatherings that may become violent", "keep a low profile" and "be aware of your surroundings".
"I subscribe to the view that we should not be criticizing our president while he is overseas, but let me say a couple of things".
It was the most recent in a series of demands and insults that critics fear will undermine a decades-old alliance launched to counterbalance Soviet aggressions.
As a result, he continued, "NATO is much stronger than it was two days ago".
Trump last week told senior aides he was going to make threats about defense spending and that he was determined to flip the table over before he left, a senior administration official said ahead of Thursday's drama, speaking on condition of anonymity to discuss sensitive summit planning. "We have a tremendous relationship with Germany", he added, saying he had raised his concerns about a new gas pipeline planned from Russian Federation to Germany. But, several sources said, Trump instead reopened in strong terms his demand that other countries spend more immediately.
Amid reports that he had threatened to pull out of the alliance due to the dispute, Mr Trump said the USA had a strong commitment.
Mr. Trump has spent weeks berating members of the alliance for failing to spend enough of their money on defense, accusing Europe of freeloading off the US and raising doubts about whether he would come to members' defense if they were ever attacked.
In response, Lindsay Koshgarian, program director for the National Priorities Project which tracks Pentagon spending and its social implications, said that European leaders-and the people they represent-should be very wary about Trump's demand to increase military expenditures. No, I don't know him well enough.
French President Emmanuel Macron said the summit atmosphere was "much calmer than everyone had said" and he saw a will to maintain the unity of the alliance, while Croatian President Kolinda Grabar-Kitarovic said Trump was "constructive". And he called alliance members "delinquent" on their contributions to NATO's budget.
White House spokesperson Sarah Sanders confirmed the ask to reporters on Wednesday, saying that the president "wants to see our allies share more of the burden and at a very minimum meet their already stated obligations". Trump has at times downplayed that conclusion, pointing to Putin's denials.