"We are doing numbers like they've never done before or ever seen before", Trump said.
Reversing the harsh criticisms he has leveled at NATO, President Trump says the alliance is very strong - in part because of promises from America's allies to boost their military budgets to 2 percent of their gross domestic product.
Trump had opened the first day of talks in Brussels on Wednesday with a public diatribe against Germany, the second biggest state in the Western defense alliance, criticizing its reliance on Russian gas imports and failure to spend more on defense.
NATO Secretary General Jens Stoltenberg, Poland's President Andrzej Duda, Greek Prime Minister Alexis Tsipras, U.S. President Donald Trump, Portugal's Prime Minister Antonio Costa, Hungarian Prime Minister Viktor Orban pose for a group photo in the park of the Cinquantenaire, during a NATO Summit, in central Brussels, Belgium July 11, 2018. "Everybody in that room got along and they agreed to pay more and they agreed to pay it more quickly".
When asked about the footage, Trump's press secretary Sarah Huckabee Sanders said Kelly was simply "displeased because he was expecting a full breakfast and there were only pastries and cheese", the Washington Post reported.
He also said that he has hope for what the allies can do to marginalize Putin, even if Trump refuses to.
Meanwhile, the Wall Street Journal reported Trump as saying "I'll do my own thing" if spending is not immediately increased.
Referring to the 97-2 vote Tuesday in the U.S. Senate affirming its support for NATO, Kerry called on Republican lawmakers to call Trump on his actions.
President Donald Trump's chief of staff, John Kelly, shifted in his seat and despondently looked away as Trump called Germany a "captive" of Russian Federation. "We're protecting everybody. And yet we're paying a lot of money to protect".
British Prime Minister Theresa May tried to set the tone on Wednesday by announcing more troops for NATO's Afghan training mission. He has also floated the idea of "going it alone" if allies don't comply, something which he has said before but he can't really do without an - unlikely- Congress approval.
Diplomats were already anxious about the summit ahead of time, not least because of an abrasive G7 meeting last month, when Mr Trump renounced a summit communique that had previously been jointly agreed.
Amid reports that he had threatened to pull out of the alliance due to the dispute, Mr Trump said the USA had a strong commitment.
Next week, he will be in Finland for a one-on-one summit with Mr. Putin, for whom he has had far kinder words than his North Atlantic Treaty Organisation allies lately.
Trump said "actual" US military spending is 4.2 percent of GDP, without explaining the big discrepancy.