NATO Summit first day overshadowed by tensions between allies

The German Chancellor also addressed the accusation of Russian influence, saying "I have experienced myself how a part of Germany was controlled by the Soviet Union. I am very happy that today we are united in freedom, the Federal Republic of Germany. Because of that we can say that we can make our independent policies and make independent decisions. That is very good, especially for people in eastern Germany."

In remarks later during the summit, however, President Trump increased pressure on allies over burden sharing, calling on allies to double defense spending targets to 4 percent, the White House confirmed.

"I think we should first get to 2 percent, focus on that now... the good thing is that we are moving to that," NATO Secretary General Stoltenberg told reporters, without speaking directly to Trump's comments.

The US president has been the source of concern for many European allies, in and out of NATO, with European Council President Donald Tusk warning Trump Tuesday to "appreciate your allies, after all you don't have that many," and reminding that "America does not have, and will not have a better ally than Europe."

The spike in tensions comes as President Trump is preparing for his first-ever summit with Russian President Vladimir Putin in Helsinki, Finland, in less than a week, sparking worry among allies.