By Robin Emmott, Jeff Mason and Alissa de Carbonnel
BRUSSELS (Reuters) - Donald Trump gave an angry ultimatum to European allies on Thursday, warning a NATO summit the United States could withdraw its support and sparking crisis talks which the U.S. president said produced big new defense spending pledges.
Other leaders, however, played down the extent to which they went beyond existing commitments to increase contributions to their own defense, as Trump demanded they share more of what he calls an unfair burden on U.S. taxpayers in funding an alliance focused on discouraging pressure from a resurgent Moscow.
In a closed-door meeting with NATO leaders, Trump said that if European governments did not spend more on defense, the United States "would have to look to go its own way", according to one diplomatic source present in the room.
Trump delivered the line after what several sources said was an improvised rant focused on his grievances about transatlantic ties, but appeared to hesitate before issuing his ultimatum, which led to some confusion about what he really meant.
French President Emmanuel Macron and others said they did not hear in Trump's warning a direct threat to quit NATO -- though the words did cause alarm -- and Trump himself later said such a move would be "unnecessary".
The early morning drama was part of two days of diplomatic theater in Brussels, as allies tried to shield a post-war world order from his "America first" demands.
It was unclear anything concrete changed, although NATO's chief Jens Stoltenberg spoke of a "new sense of urgency".
A month after he walked out of a G7 economic summit amid rows about new U.S. tariffs that have provoked fears of global trade war, Trump was already at the center of a storm from the start of the NATO summit on Wednesday.
He accused German Chancellor Angela Merkel of being beholden to Moscow due to energy imports, while letting Americans pay for protecting Germany from Russia.
That tension seemed to calm during a gala dinner but that did not last. Trump tweeted out more of his anger overnight, saying billions of dollars in new spending by NATO allies since last year "isn't nearly enough".
When the summit resumed for a session with the leaders of non-members Ukraine and Georgia, Trump failed to appear for nearly an hour, officials said. And when he did, he soon used his turn to speak to stray from the scheduled topic and to return to his budget complaints in even stronger language.
People present said he raged that allies, notably Germany, did not make vast increases in their defense budgets. Pledges to spend 2 percent of national income on defense by 2024 must be met by January, he said -- a dizzying idea for many countries which currently spend just half that.
Another NATO diplomat said Trump trampled on protocol by pointing at some leaders he said were not spending enough and addressing Merkel by her first name, referring to her as "you, Angela".
Stoltenberg, stepping in -- not for the first time -- as peacemaker between Trump and the other 28, called a special closed-door session, the first in a decade, with most officials and the invited guests ushered out, to allow the alliance's principal leaders to remonstrate with Trump.
Merkel, facing domestic political opposition to pushing defense spending up from 1.2 percent of GDP, said she explained to Trump how much was already being done. NATO has spent an extra $90 billion on defense since 2015, after Russia seized Crimea from Ukraine in 2014.
Macron said he had just agreed a 2019 budget with parliament so changing it was unrealistic -- a point Trump later said he had accepted though he still expected all members to hit the 2 percent target in the next years, and then possibly double that.
"I let them know that I was extremely unhappy," an ebullient Trump told reporters afterwards. But he added that the talks had ended on good terms: "Everybody in that room got along and they agreed to pay more and they agreed to pay it more quickly."
In a characteristically freewheeling news conference at NATO headquarters, covering his impending visit to Britain, talks with Russian President Vladimir Putin on Monday, Iran, China, and his father and mother's European roots, Trump also returned to a favored theme. He linked calls for higher defense spending to complaints about Germany's trade surplus and renewed a threat to raise tariffs on EU-made cars if trade terms do not change.
Stoltenberg, a former Norwegian premier who Trump said gave him "total credit" for a successful summit, told reporters: "We had a very frank and open discussion ... That discussion has made NATO stronger. It has created a new sense of urgency."
Merkel was among those, however, who gave little indication that anything concretely new had been pledged by those present.
"The American president demanded what has been discussed for months, that there is a change in the burden-sharing. I made clear that we are on this path," she said, a day after having to challenge Trump's suggestion German imports of Russian gas meant that her country was "totally controlled by Russia".
Macron said France, which last year spent 1.8 percent on defense, would meet the target by the 2024 deadline.
For many of those present, Trump's demands that they move closer to the 3.6 percent of GDP Washington spends on the world's most powerful military make little sense. "Even if we had the money, what would spend it on?" one NATO diplomat said.
"In the case of Germany, a lot of European countries would be very uncomfortable with that level of spending," the diplomat added -- a nod to the World War Two aggression that was to lead to NATO's creation. "It would be armed to the teeth."
(Additional reporting by Sabine Siebold, Phil Stewart and Humeyra Pamuk in Brussels and John Walcott in Washington; Writing by Alastair Macdonald; Editing by Peter Graff and Catherine Evans)