"The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable. While these countries have been increasing their contributions since I took office, they must do much more,” Trump wrote on Twitter

In another tweet, the Republican president complained about Europe’s trade surplus with the United States. Trump has slapped tariffs on billions of dollars worth of steel and aluminum imports from China, the European Union, Canada and others, prompting retaliation against U.S. products. He is considering extending the levies to the auto sector.

Trump has been a harsh critic of NATO since the 2016 presidential campaign. After taking office, he affirmed the U.S. commitment to the defense of NATO allies but kept up complaints about Washington shouldering the financial burden.

By current standards, Washington funds about 70 percent of NATO spending. NATO members have agreed to spend 2 percent of economic output on defense every year by 2024, but Germany and Spain are among the countries not expected to meet the target.

At the first NATO summit he attended last year, Trump denounced Europe’s low defense spending and called for NATO to be on the front line against Islamic militants. He also surprised many NATO leaders by urging them to focus on illegal immigration.

This year, trade is on his mind.

"I’m going to tell NATO: You’ve got to start paying your bills. The United States is not going to take care of everything,” Trump said at a rally last week. "They kill us on trade.”

Founded to deter the Soviet threat in 1949, NATO is based on deep cooperation with the United States, which provides for Europe’s security with its nuclear and conventional arsenals.

NATO has found renewed purpose since Russia’s 2014 annexation of Crimea, sending battalions to the Baltics and Poland to deter potential Russian incursions.