NATO Secretary General Jens Stoltenberg (center L) shakes hands with U.S. President Donald Trump during a NATO summit in Brussels, Belgium, July 11, 2018. NATO leaders gather in Brussels for a two-day meeting. (Xinhua/Ye Pingfan)

WASHINGTON, July 11 (Xinhua) -- U.S. President Donald Trump on Wednesday urged members of the North Atlantic Treaty Organization (NATO) to "immediately" meet their two-percent GDP benchmark on defense spending.

Trump, who was in Brussels to attend a two-day NATO summit, complained in a tweet about why there are "only 5 out of 29 countries that have met their commitment?"

"The U.S. is paying for Europe's protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025," the U.S. president said in the tweet.

Trump at the summit also told NATO leaders they should increase their defense spending to four percent of their country's GDP (gross domestic product), a White House official said on Wednesday.

Trump has long been ripping the NATO countries that have not met the two-percent benchmark on military spending.

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 two percent of their GDP on defense.

Eight NATO allies will reach the benchmark by the end of the year, and 15 are on the track of meeting the target by 2024, according to a recent statement by the White House.