U.S. President Donald Trump speaks during a press conference on the second day of the NATO Summit in Brussels, Belgium, on July 12, 2018. (Xinhua/Ye Pingfan)

U.S. President Donald Trump said he won big concessions from other member of the North Atlantic Treaty Organization (NATO) over boosting defense spending on Thursday, at an unscheduled press conference at the end of NATO summit.

"Yesterday I let them know that I was extremely unhappy with what was happening and they have substantially upped their commitment," Trump said without elaborating.

"Everyone has agreed to substantially up their commitment. They're going to up it at levels that they never thought of before," Trump said, adding that "now we're very happy."

Before the summit, other NATO members have committed to increase their defense budgets.

Trump reaffirmed U.S. commitment to NATO, saying the alliance is "very unified, very strong, no problem."