After meeting with Trump at the EU, European Council president Donald Tusk said he and the USA president agreed on the need to combat terrorism but some differences loomed large.
Trump said that 23 of 28 member nations do not pay what they should for their defense and pointed out that the US has spent more than all other member nations combined over the last eight years.
"I am not 100 percent sure that we can say today - "we" means Mr. President and myself - that we have common position, common opinion about Russia", Tusk said.
"There were signs, too, that Mr. Trump and the allies remain at odds over Russia, which is deeply unsettling given mounting questions about Russian interference in the 2016 presidential election".
The 27 other leaders looked on in awkward silence as Trump suggested most North Atlantic Treaty Organisation countries were freeloaders not paying their share for military protection.
Mr Trump's sharp words may have won him few friends among the gathered leaders, who appeared to avoid Mr Trump at the end of the "family photo", the group photograph customary at these global summits. Most NATO members, meanwhile, spend less than the 2 percent benchmark.
US President Donald Trump makes his move and overtakes the prime minister of Montenegro.
Trump announced a review of the "deeply troubling" leaks and warned that those responsible could face prosecution, the White House said. At another, Trump seemed to forcibly grab Macron, who had been avoiding him, for a handshake.
He said Trump had a few days ago signed a U.S. budget proposal to hike spending on United States forces in Europe by 40 percent, in what Stoltenberg called "the best demonstration of commitment to the alliance".
"They agreed on the need to work together to deepen our security cooperation in fighting ISIS (Islamic State), combating radicalisation and responding to other common threats".