"Twenty three of the 28 member nations are still not paying what they should be paying and what they're supposed to be paying for their defence", he said.
In the past eight years, the United States spent more on defense than all other North Atlantic Treaty Organisation countries combined, Trump said.
The delicate manner in which she did so serves to highlight the political dilemma for the United Kingdom leader, who raised the national terror threat to the highest "critical" level two weeks from a general election: May needs Trump more than ever as she seeks to forge a new place for Britain outside the European Union.
"We saw two different Trumps", one European Union official said, describing campaign comments about the benefits of Britain's exit and probability of further departures as "the Nigel Farage era" when the UK Independence Party leader had Trump's ear, and saying that was followed by a "more pragmatic and realistic" approach once Trump was installed in the White House. Former Secretary of State Madeleine Albright tweeted that the USA can not "add an asterisk" to Article 5.
All member states have previously pledged to work toward spending 2 percent of their GDP on defense.
Gen. Jens Stoltenberg says the military alliance is lending its support to the USA -led coalition against the group calling itself the Islamic State of Iraq and the Levant.More news: Jared Kushner now the focus in Russia, Trump investigation
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The president pledged to "get to the bottom" of alleged leaks coming out of US government agencies, saying in a written statement he was asking the Department of Justice and other relevant agencies to launch a "complete review". An anti-terror coordinator may also be named.
Yet NATO leaders are keen to show that the alliance born in the Cold War is responding to today's security threats. What the Sicily stay will likely not offer: a news conference, as Trump appears set to defy presidential tradition and not hold one during the entire trip.
White House spokesman Sean Spicer, asked about Trump moving past Markovic, said he had not seen the video but assumed the USA president was moving to his designated spot.
The U.K.is furious that American officials gave away the name of the suspected suicide bomber, and images of his improvised explosive device to US media, while British police race to prevent further attacks from the same network.
President Trump has made it abundantly clear that he likes to constantly assert his dominance. "And I am not 100 percent sure that we can say today - "we" means Mr. President and myself - that we have a common position, common opinions about Russian Federation", said Tusk.
Earlier in the day, the handshake with newly-elected French president Emmanuel Macron was described by the pool reporter as having been "so long that knuckles started to turn white".