"What they're doing is spending - at a much faster clip they're going up to the 2 percent level", Trump said.
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Merkel said she welcomed the chance to have an "exchange of views" with Trump.
Hundreds of demonstrators chanted outside the USA ambassador's residence where Trump was staying on the outskirts of London, providing a preview of the forceful protests expected during his two-day British stay. It's also always been a bugbear for the United States, which spends more on defence both as a percentage of GDP and in absolute terms than any other member.
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Trump has said next week's meeting with Putin "may be the easiest" part of his European tour. But with Trump scheduled to meet with President Vladimir Putin of Russian Federation on Monday, the gathering was dominated by wrangling over money.
"We are being played for fools", he reportedly told leaders at a meeting meant to revolve around Georgia and Ukraine.
He said North Atlantic Treaty Organisation chief Jens Stoltenberg had given "total credit" to him for the increased spending, which he said he instigated during his first North Atlantic Treaty Organisation meeting previous year.
A new poll shows that a plurality of Germans favor a drawdown of USA troops, and only 15 percent believe that Germany should raise its defense spending to 2 percent of GDP.
Stoltenberg acknowledged that Trump had expressed himself in "very direct language" but insisted that away from the fiery rhetoric the allies all agree on fundamental issues: the need to boost NATO's resilience, fight terror and share the cost of defence more equally.
"We have a very, very good relationship with the chancellor".
Trump threw the summit into turmoil by demanding not only that allies reach their commitment to increase spending to two percent of GDP "immediately" - instead of by 2024 as previously agreed - but also telling them to eventually double the figure to a punishing four percent. And he said Canada has promised to reverse a decline in military resources with an eye towards the two per cent target. It will be Trump's first working visit to the United Kingdom since he became President, and massive protests in London are expected.
The U.S. president's shake-up of the NATO summit had begun Wednesday at a working breakfast with NATO Secretary General Jens Stoltenberg, when Trump accused Germany of being controlled by Russian interests. Trump wants the spending to go up to 4% of GDP, which is more than even the USA now spends on defense.
Trump did not directly answer questions about whether he had threatened to take the U.S. out of the alliance, saying he "told people I'd be very unhappy if they didn't up their commitments very substantially".