File Photo: King Philippe of Belgium, NATO Secretary General Jens Stoltenberg, U.S. President Donald Trump, British Prime Minister Theresa May (from L to R, front) and other NATO member states leaders watch air show at the handover ceremony of the new NATO headquarters during a one-day NATO Summit, in Brussels, Belgium, May 25, 2017. (Xinhua/Ye Pingfan)

WASHINGTON, July 9 (Xinhua) -- U.S. President Donald Trump on Monday complained again about the defense expenditure of the North Atlantic Treaty Organization (NATO) members, one day ahead of his departure for a NATO summit scheduled in Brussels, Belgium.

"The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable," tweeted Trump on Monday morning.

"While these countries have been increasing their contributions since I took office, they must do much more," Trump added.

The U.S. president has long been scolding the NATO countries which have not met the 2-percent-of-GDP benchmark on military spending. It has also been seen as a key topic for the upcoming NATO summit slated for July 11-12.

"NATO benefits Europe far more than it does the U.S.," Trump claimed.

NATO's European allies have spent more on defense for a second consecutive year in 2017 but the majority of them still failed to meet the target of spending 2 percent of their gross domestic product on defense.

Eight NATO allies will reach the 2 percent benchmark by the end of this year, and 15 are on track to spend 2 percent by 2024, according to a statement released by the White House recently.

In late May, Trump threatened that NATO member states who have not met their financial obligations regarding defense will be "dealt with," and specifically called upon Germany to fulfill its obligation.