Donald Trump has been ridiculed on social media for appearing to push Montenegro Prime Minister Dusko Markovic out of the way to get to the front of a group of North Atlantic Treaty Organisation leaders.
"I'm not 100 percent sure that we can say today - "we" means Mr President and myself - that we have a common position, common opinion about Russian Federation", former Polish prime minister Tusk said.
But he instead returned to a grievance about Europe's drop in defence spending since the end of the Cold War and failed to publicly commit to NATO's founding Article V rule which stipulates that an attack on one ally is an attack against all.
Stoltenberg refused to be drawn into a row between the United States and Britain after leaked photos from the Manchester bomb scene appeared in The New York Times.
As a presidential candidate, Trump railed against NATO's financial burden-sharing, suggesting the USA might only come to the defense of countries that meet the alliance's guidelines - for committing 2 percent of their gross domestic product to military spending.
At Nato's gleaming new headquarters, Trump returned to his longstanding call for member nations to pay their fair share, lecturing leaders about contributing more as they stood listening in awkward silence.
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Trump is slated to leave Brussels late Thursday for the final piece of his trip, a two-day stay in Sicily for G-7 meetings.
Trump said that the United States had spent more on defence in the last nine years than all the other members together.
In another crucial signal, Trump refused to reaffirm the value of NATO's Article 5 collective defense clause, the linchpin of NATO's unity and deterrence since the alliance's foundation in 1949.
Secretary-General Jens Stoltenberg said that joining the USA -led anti-IS coalition "will send a strong political message of NATO's commitment to the fight against terrorism and also improve our coordination within the coalition".
"If you are standing at a ceremony talking about the invocation of Article 5 after 9/11 and talking about that, that is a pretty clear indication of the support that exists for it", Spicer said. The secretary general now wants countries to present plans on how they'll reach that level of spending.
In one nod to Trump, NATO leaders are due to agree later on Thursday for the Western military bloc to join the US -led, 68-nation coalition against Islamic State in Syria and Iraq.
The aftermath of that attack in Manchester, England, has produced further tension, as a British official said that police have decided not to share further information on the investigation due to leaks blamed on USA officials.