Donald Trump has claimed a personal victory at a NATO summit after telling European allies to increase spending or lose Washington's support, an ultimatum that forced leaders to huddle in a crisis session with the US president.
Trump emerged declaring continued commitment to a Western alliance built on US military might that has stood up to Moscow since World War II.
People present said he had earlier warned he would "go it alone" if allies, notably Germany, did not make vast increases in their defence budgets for next year.
"I let them know that I was extremely unhappy," he said, but added that the talks ended on the best of terms: "It all came together at the end. It was a little tough for a little while."
German Chancellor Angela Merkel, who called the summit "very intense", and other leaders including French President Emmanuel Macron, played down the extent to which they had pledged to accelerate spending plans as fast as Trump wanted.
"He said they must raise spending by January 2019 or the United States would go it alone," one person said of the clash at NATO headquarters when Trump spoke in a debate that was meant to move to other matters after rows over spending on Wednesday.
Macron and others said they did not interpret Trump's words as a direct threat to quit the alliance Washington founded in 1949 to contain Soviet expansion. Trump, asked if he thought he could withdraw from NATO without backing from Congress, said he believed he could but it was "unnecessary".
Others say Congressional approval would be required - and would be unlikely to be forthcoming.
Trump hailed a personal victory for his own strategy in complaining loudly that NATO budgets were unfair to US taxpayers, and the emergence of what he said was a warm consensus around him.
Several diplomats and officials said, however, that his undiplomatic intervention - including pointing at other leaders and addressing Merkel as "you, Angela" - had irritated many.
As the drama unfolded, a day after Trump launched a virulent public attack on German policy, NATO Secretary General Jens Stoltenberg cleared the room of many officials and the invited leaders of non-members Georgia and Afghanistan so that the other 28 leaders could hold a closed session with the president.
NATO members have committed to spending at least two per cent of their national income on defence by 2024, though the terms allow for stretching that in some cases to 2030. The United States, far the biggest economy, spent 3.6 per cent last year, while Germany, the second biggest, paid out just 1.2 per cent and only a handful of countries met the 2 per cent target.
Australian Associated Press