Nato leaders held an emergency meeting on Thursday after US President Donald Trump demanded that members immediately increase and ultimately double their defence spending.
Nato secretary general Jens Stoltenberg "has therefore convened a meeting in Atlantic Council format, which involves all 29 countries in the alliance", a source said.
Mr Trump said after the meeting that his demands had made Nato stronger than ever. Speaking at a news conference at Nato headquarters in Brussels, Mr Trump said the alliance was now “very unified” after members reacted positively to his demand for greater contributions at the meeting on spending.
His statement came after reports that he had threatened to “do his own thing” and hinted at pulling the US out of the alliance.
“The people have stepped up today like they have never stepped up before. We had a really great meeting today. Everybody in that room ... agreed to pay more, and they agreed to pay it more quickly,” he said.
“Yesterday, I let them know that I was extremely unhappy with what was happening,” he said. “Now we have a strong Nato. Much stronger than it was two days ago.”
German Chancellor Angela Merkel said told reporters that "there was a clear commitment to Nato by all" at an emergency session of the military alliance.
The US president requested allies increase their spending to 2 per cent of GDP immediately, instead of by 2024, and eventually double spending to 4 per cent.
Mr Trump made his demand public in a tweet on Thursday, confirming what he reportedly told Nato leaders at a private dinner in Brussels on Wednesday night.
“All Nato nations must meet their 2 per cent commitment, and that must ultimately go to 4 per cent,” the president said.
His call for a doubling of defence budgets came even though only five of Nato's 29 members currently meet the 2 per cent threshold agreed upon by the transatlantic alliance in 2014. Poland just misses the target, spending 1.98 per cent of its GDP on defence.
The US leader wrote in an earlier tweet: “Billions of additional dollars are being spent by Nato countries since my visit last year, at my request, but it isn’t nearly enough. US spends too much. Europe’s borders are BAD!”
He had said earlier in the day that he expected alliance members to increase their contributions “immediately, not by 2025”, making reference to the wrong year intended for that target to be met.
The billionaire property magnate had condemned European allies over their contributions on social media in the buildup to the summit and in front of cameras when he arrived in the Belgian capital. But, by all accounts, he was more restrained and friendly at the formal dinner hosted at the Art and History Museum at the Cinquantenaire, despite tweeting “what good is Nato” just minutes before his arrival.
"He was in a good mood, he said Europe was a continent he appreciated," Luxembourg's Prime Minister Xavier Bettel said. "We didn't know what to expect, it was a positive outcome."
But Mr Trump's demand for such a vast amount of money to be pumped into the alliance and defence of Europe shocked observers and leaders alike.
“This is insane. Trump is officially calling for most of Europe to more than double their defence budgets – which would be an enormous militarisation of Europe well beyond even what the United States spends on defence,” Brian Klaas, political scientist at the London School of Economics, said on Twitter.
Canadian Prime Minister Justin Trudeau said on Tuesday, before Mr Trump’s new demand, that Ottawa would not double its contribution from 1 per cent to 2 per cent.
On Wednesday, Mr Trump angered Germany by saying it was "captive" to Russia because of its energy supply from Russia. It shattered any hopes that officials here had of a show of transatlantic unity.
The US president then met is Polish counterpart Andrzej Duda to discuss strengthening military cooperation. Warsaw has called for US troops to be stationed in Poland permanently to protect against Russia. The meeting about Russia was in line with Mr Trump’s tactic of criticising countries who co-operate with Moscow, particularly Germany.
On Thursday, he tweeted again about German energy ties to Russia. "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."
The second day of the summit is set to focus on the alliance’s mission in Afghanistan, with member states expected to recommit their resources. Nato also insisted that Georgia will one day join the alliance, in spite of separatist ambitions in sections of the country.
"Georgia will become a member of Nato," said Mr Stoltenberg.
Mr Trump will hold talks with the leaders of Ukraine, Georgia, Romania and Azerbaijan in Brussels before travelling to Britain for his first official visit as US president and then to Helsinki, Finland, where he has organised a one-on-one summit with Russian President Vladimir Putin just days after the Nato summit. As he left for Brussels, he had said that it would be his "easiest" stop.
Mr Stoltenberg is again expected to attempt to portray a united front within Nato, and sidestep any questions about Mr Trump’s new 4 per cent demand, as he did on Wednesday.
“We have agreed that we're committed to the pledge of increasing defense spending to 2 per cent," he said after a meeting with Mr Trump. "So let's start with that.”