"The US were not treated fairly but now we are. I believe in NATO," Trump told a press conference after a fraught NATO summit in Brussels.

"The US commitment to NATO remains very strong," Trump added, "mainly because (of) the additional money they've committed."

Trump's comments came amid reports that he had warned leaders at an emergency session that the United States could pull out of the alliance if other members failed to increase their contributions.

Trump said there had been "tremendous progress" and that allies were "going to up it at levels they've never seen before."