NATO Summit first day overshadowed by tensions between allies

According to the latest NATO estimates, Germany spends 1.24 percent of GDP on defense, while the US spends 3.5 percent.

The claims stem from criticisms about burden-sharing that Trump has made since just prior to his inauguration in January 2017. In addition to referring to NATO as "obsolete" at the time, he blasted allies for not meeting 2 percent defense spending commitments.

Since his inauguration, Trump has argued that fellow NATO allies have been leaving the United States to foot the bill for their joint defense. According to NATO figures, only five of the 29 members have met defense spending targets this year: Estonia, Greece, Latvia, Britain and the United States.

German Chancellor Angela Merkel hit back at Trump just a few hours after his remarks, arriving at NATO headquarters to say that Germany had contributed a lot to the defence alliance.

"Germany is the second largest provider of troops, the largest part of our military capacity is offered to NATO and until today we have a strong engagement towards Afghanistan," Merkel said, adding: "In that we also defend the interests of the United States."