"The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable," Trump tweeted a day before he departs for the summit, which will take place July 11-12 in Brussels.

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

Trump has repeatedly criticized NATO states for allegedly not pulling their financial weight, and has been reaching out to Russian President Vladimir Putin even as his ties with some of his western allies have become strained.

Diplomats fear a rancorous meeting in Brussels could undermine efforts to show unity in the face of the growing threat on the alliance's eastern flank, particularly with Trump set to meet Putin in Helsinki a few days later.

Trump has insisted that he will be "totally prepared" for the summit with Putin, saying that he "might even end up having a good relationship" with the Russian strongman.