"We're supposed to protect Germany but they're getting their energy from Russian Federation", he said.
As a result, he continued, "NATO is much stronger than it was two days ago".
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 USA taxpayers, funding a defense budget worth about 3.6 percent of their national income, foot much of NATO's bills.
All 29 North Atlantic Treaty Organisation leaders including Trump backed a joint statement committing themselves to greater "burden sharing" and to the alliance's founding commitment that an attack on one member is an attack on them all - with no mention of the four per cent.
"I think I can probably can [pull out of NATO], but that's unnecessary, and the people have stepped up today like they've never stepped up before", Trump added.
Duda said he would be "shocked" if Trump made any move would decrease the region's security.
It was only last month that after a disastrous G-7 summit, Trump signed a joint communiqué with other allies and then changed his mind while flying home.
Sources familiar with the first round of closed-door talks on Thursday told the Reuters news agency that Mr. Trump carried his firm stance on military spending from Twitter into the meeting rooms.
Syria thwarts Israeli strike on Quneitra
Israel launched a Patriot missile at a drone approaching Israeli territory from Syria, the country's military said on Wednesday. The Golan Heights was captured by Israel amid war in 1967 and illegally annexed in 1981.
Meanwhile, the Wall Street Journal reported Trump as saying "I'll do my own thing" if spending is not immediately increased.
Focusing on numbers, rather than capabilities, "is an inadequate way to measure allies' impact and their various contributions to NATO's mission of collective defense".
Mr Trump had opened the first day of talks in Brussels yesterday with a public diatribe against Germany, the second biggest state in the Western defence alliance, before the mood appeared to have calmed as the summit went into its second day, focusing on operations beyond Europe.
In fact, the 29 members each pay a portion of NATO's annual US$3-billion operating budget based on the size of their economies.
"Dear America, appreciate your allies, after all you don't have that many", Tusk said, before reminding Trump that European troops had fought alongside Americans in Afghanistan following the September 11, 2001 attacks on the United States.
The meeting brought together Mr Trump and Canada's prime minister Justin Trudeau, who irritated the U.S. president at the G7 by declaring that his country would not be pushed around.
Stoltenberg acknowledged that Trump had expressed himself in "very direct language" but insisted that away from the fiery rhetoric the allies all agree on fundamental issues: the need to boost NATO's resilience, fight terror and share the cost of defence more equally.