Trump's comments will disappoint alliance leaders who had hoped for a public commitment from the US president to NATO's security guarantees, which he called into question during his presidential campaign previous year.
The act was being widely denounced online.
"This will send a strong political message of Nato's commitment to the fight against terrorism", Nato Secretary General Jens Stoltenberg told reporters ahead of Trump's meeting with the alliance's other 27 leaders.
The rebuke was being criticized by opponents.
"It is natural that the President of the United States is in the front row".
Trump added insult to injury by claiming the Canadians and Europeans "owe massive amounts of money from past years and not paying in those past years".
Other NATO leaders wanted him to "express a commitment to Article 5 of the NATO charter, which says that an attack on one nation is an attack on all nations", Tamara reports.More news: DeVos to Review Rejected College Prep Grant Applications
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Trump also unveiled a memorial dedicated to the victims of the 9/11 attacks.
President Donald Trump opened meetings with European Union leaders yesterday against the backdrop of striking anger from Britain over intelligence leaks and a decision by Manchester police to withhold information from the United States about the investigation into this week's bombing.
Further protests were planned on Thursday, when Trump will first meet with EU President Tusk and Commission chief Juncker at the bloc's headquarters."The NATO of the future must include a great focus on terrorism and immigration, as well as threats from Russian Federation and our nation's eastern and southern borders", Trump said. Most NATO members, meanwhile, spend less than the 2 percent benchmark.
President Trump is in Italy Friday for meetings with leaders of the biggest economies in the world.
In unexpectedly abrupt remarks as North Atlantic Treaty Organisation leaders stood alongside him, Trump said certain member countries owed "massive amounts of money" to the United States and North Atlantic Treaty Organisation - even though allied contributions are voluntary, with multiple budgets.
British officials are particularly angry that photos detailing evidence about the bomb used in the Manchester attack were published in The New York Times, although it's not clear that the paper obtained the photos from US officials. "The Trump brand of diplomacy has some analysts anxious that the nine-day tour of allies in the Mideast and Europe might only make his troubles worse", wrote the Washington Post before the trip began. "Every president has reaffirmed collective defense and today was the day for him to do it".
President Donald Trump is at the Vatican to meet with Pope Francis, the famously humble pontiff with whom he has publicly clashed.