"Must pay 2% of GDP IMMEDIATELY, not by 2025", he tweeted Wednesday.
As well as Merkel, Spanish Prime Minister Pedro Sanchez and Charles Michel, the prime minister of Belgium, were singled out by Trump for undershooting on their spending targets when US taxpayers, funding a defense budget worth about 3.6 percent of their national income, foot much of NATO's bills.
He also said he thought spending of 4 percent on defense - similar to the U.S. level - would be the right level.
Other leaders, however, played down the extent to which they went beyond existing commitments to increase contributions to their own defence, as Trump demanded they share more of what he calls an unfair burden on USA taxpayers in funding an alliance focused on discouraging pressure from a resurgent Moscow.
Trudeau said he did agree to uphold Canada's commitment to the 2014 Wales NATO summit pledge on defence investment, but took pains to point out the declaration technically states NATO allies would merely "aim to move towards" the two per cent guideline within a decade.
Trump had pushed for an increase to 4 percent - about what the U.S. spends.
At NATO, Trump did not specify which countries had committed to what, and it remained unclear whether any had changed their plans. "Nato is very important but it's helping Europe far more than it's helping us".
Despite that, Stoltenberg had been forced to call an extraordinary session of all 29 allies to discuss Trump's demands, in what officials said was one of the first of its kind at a North Atlantic Treaty Organisation summit for a decade. "There is a great, great spirit leaving that room".
The U.S. President was also asked if he would be prepared to recognise Crimea as part of Russian Federation after it was annexed from Ukraine by Moscow in 2014.More news: Trump greeted with protests during first United Kingdom visit
Trump also said he would once again raise the sensitive issue of election meddling with Putin - then predicted his counterpart 'may deny it'.
"I think we go into that meeting not looking for so much, ' Trump said".
"I felt, we all felt, that we needed more time for a discussion on burden sharing", Stoltenberg explained to reporters afterward - even though the summit statement had been signed the previous day.
President Kolinda Grabar-Kitarovic mounted a charm offensive at the summit, presenting Trump with a Croatian soccer jersey as giving him plenty of attention. Instead, they said, members reaffirmed the need to stay on track with military-budget increases that have already been underway and were branded insufficient by Trump a day earlier.
After a week in which he had insulted allies and questioned the value of NATO, Trump-flanked by Secretary of State Mike Pompeo and National Security Adviser John Bolton-gave an impromptu news conference at NATO headquarters Thursday morning before leaving Brussels.
According to CNN, he said: "I told people that I would be very unhappy if they didn't up their commitments very substantially". And we made clear, well, I made clear for myself and others also made it clear, that we are on this path, which is in our own interest and can make us overall stronger.
Though she said that Germany did have "a lot of issues with Russian Federation", she noted that it was important to "keep the communication line between countries or alliances and opponents" open.
"I believe through strength you get peace", he said.