WASHINGTON -- U.S. President Donald Trump repeated criticism of NATO allies and their defense spending, days before he attends a crucial summit of heads of state from the North Atlantic alliance.

Like past U.S. presidents, Trump has voiced frustration that the United States is the largest contributor of defense spending for NATO.

But Trump has taken his criticism much further, both during his election campaign and now in the White House. He also questioned the NATO's most important feature: that an attack on one member should be considered an attack on all members.

"The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable," he said in series of Twitter posts on July 9.

"While these countries have been increasing their contributions since I took office, they must do much more," he wrote. NATO benefits "Europe far more than it does the US," he said.

NATO members are committed to spending 2 percent of their gross domestic product on defense by 2024, but only a handful of countries have met or come close to that target.

Combined with the building trade war between Washington and some of its closest North American and European partners, Trump's comments suggest that the Brussels meeting may be tumultuous.

Days after, Trump heads to Helsinki to meet Russian President Vladimir Putin, a meeting that has also worried U.S. allies.