Baltic leaders fearful of any repeat of Russia's annexation of Crimea called for unity as they arrived at the summit, while Slovakia's President Andrej Kiska said his country was "one of the good guys" because he was increasing defense spending.
During the president's remarks, he suggested countries not only meet their minimum commitment of 2 percent but increase it to 4 percent, Sarah Huckabee Sanders, White House press secretary said in a statement.
Sitting at the table with the president were Secretary of Defense James Mattis, Chief of Staff John Kelly, Secretary of State Mike Pompeo and NATO Ambassador Kay Bailey Hutchison, Trump defended his public reprimanding of allied countries over their defense spending.
TRUMP: "Because of me, they've raised about $40 billion over the a year ago." - remarks about NATO-member spending on defense at breakfast with Stoltenberg.
Merkel later responded to Trump's remarks, saying Germany, one of the biggest troop contributors to North Atlantic Treaty Organisation missions, was free of Russian control since the fall of the Berlin Wall. "The good news is that we are making progress". We're stronger together than apart.
Kerry, who served as the Secretary of State from 2013-17, said Trump makes public adversaries out of its friends, and turns USA adversary, who has been attacking America's democracy, into his fawned-over ally.
Trump is also expected to attend the first official summit and a one-on-one meeting with Putin in Helsinki, Finland, next week.
As the day began, Trump wasted no time going on the offensive as he began a week of high-stakes diplomacy on both sides of the former Cold War divide.
And it comes just days before Trump sits down with Putin in Finland at the conclusion of his closely watched European trip.
"Germany goes out and pays billions and billions of dollars a year to Russian Federation", said Trump.
"They will use that to weaken the European Union, to divide the European allies", Shevchenko said in an interview. "If we look at the gas pipeline, Germany is an independent country".
German Chancellor Angela Merkel pushed back firmly, insisting that Germany makes its own decisions and drawing on her own background growing up in communist East Germany behind the Iron Curtain.
Trump has preferred to take aim at allies.
Despite the friendly rhetoric, U.S. -Russia relations have soured since Trump's presidency. Opponents state that Trump is furthering Putin's agenda to break up the Western alliance.
Trump's pipeline criticism was an unusual line of attack for a president who has proclaimed himself eager to improve relations with Russia's Vladimir Putin and dismissed the USA intelligence community's assessment that Russian Federation tried to undermine Western democracy by meddling in the 2016 US presidential election to help Trump win.
"I welcome President Trump's forthcoming meeting with President Putin: open channels of communication between the United States and Russian Federation are key to managing the risks of confrontation", she planned to say, according to the spokesman.