(From L to R, first row) German Chancellor Angela Merkel, Belgian Prime Minister Charles Michel, NATO Secretary General Jens Stoltenberg, U.S. President Donald Trump and British Prime Minister Theresa May attend the opening ceremony at the 2018 NATO Summit at NATO headquarters on July 11, 2018 in Brussels, Belgium.
Trump has taken particular aim at the proposed Nord Stream II gas pipeline, which is set to run from Russian Federation to Germany under the Baltic Sea.
It was the most recent in a series of demands and insults that critics fear will undermine the decades-old alliance, coming days before Trump sits down with Putin at the conclusion of his closely watched European trip.
During the summit, Trump reportedly broke diplomatic protocol by calling German Chancellor Angela Merkel by her first name, saying "Angela, you need to do something about this", a source told Reuters.
Merkel, who grew up in communist East Germany, shot back that she had "experienced myself a part of Germany controlled by the Soviet Union, and I'm very happy today that we are united in freedom as the Federal Republic of Germany and can thus say that we can determine our own policies and make our own decisions and that's very good".More news: Man With World's Longest Fingernails Cuts Them After 66yrs
The U.S. president has been the source of concern for many European allies, in and out of North Atlantic Treaty Organisation, 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".
Trump will meet the Russian leader in Helsinki on 16 July for their first summit amid an ongoing investigation in the United States into possible Trump campaign collusion with Russia.
But with tensions in the Western alliance smouldering over Trump's trade tariffs on European steel and his demands for more contributions to ease the burden on U.S. taxpayers, his earlier remarks fuelled concerns among allies for the USA role in keeping the peace that has reigned since World War II.
Merkel said it was very important to have such exchanges like the meeting she and Trump were having because "after all, we are partners".
But she insisted there must be "unity and strength" in the military alliance and he must not skirt issues like the Salisbury nerve agent attack on former spy Sergei Skripal and daughter Yulia.More news: Andrew Benintendi Comes Up Short In MLB's All-Star Final Vote
Trump heads to Britain on Thursday, where the government is in crisis over Brexit and where tensions with Russian Federation have spiked after London blamed Moscow for the death this month of a British woman from contact with the Novichok nerve agent. "He may be the only one, but that's OK with me".
Funding has averaged about $1 billion annually and Stoltenberg has said he expected that level to be met.
But Trump has been more conciliatory behind the scenes, including at a leaders' dinner Wednesday.
U.S. officials have told Reuters that Washington is preparing another review of strategy, a year after Trump begrudgingly agreed to extend involvement in the 17-year-old war. He authorized the deployment of an additional 3,000 troops, bringing the total to around 15,000.More news: Stormy Daniels Arrested In Ohio Strip Club, Michael Avenatti Says