President Donald Trump speaks with the news media before boarding Marine One for travel to Europe from the White House, in Washington, U.S., July 10, 2018.

President Donald Trump is setting the tone before his meetings with European leaders this week, beginning with the NATO summit in Brussels.

"NATO countries must pay MORE, the United States must pay LESS," Trump said in a tweet Tuesday. Later in front of the White House, he told reporters that "NATO has not treated us fairly but I think we'll work something out."

@realDonaldTrump: NATO countries must pay MORE, the United States must pay LESS. Very Unfair

"The U.S. is spending many time more than any other country in order to protect them" and it's "not fair to the U.S. taxpayer," Trump said in another tweet.

And the president linked that spending to his trade battles, writing, "Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"

@realDonaldTrump: Getting ready to leave for Europe. First meeting - NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)

Trump will face fellow leaders on Wednesday and Thursday at the NATO summit in Brussels, where defense spending is expected to be a thorny subject of discussion. Ahead of the NATO summit in Brussels this week, he has repeatedly blasted the alliance’s members for not meeting a 2014 pledge to spend 2 percent of their gross domestic product (GDP) on defense.

Claiming that other countries are not pulling their weight, although the contributions are only voluntary, Trump has previously warned that the U.S. would not to be willing to intervene on the behalf of nations that did not reach the 2 percent spending target.

NATO Secretary General Jens Stoltenberg conceded that there were “some real disagreements” between the U.S. and other NATO members over trade, climate change and the Iranian nuclear deal. He told CNBC last week that NATO had previously been successful in overcoming those disagreements. “I am confident we will do so again this time,” he said.

NATO’s 2017 report noted that “European allies and Canada increased spending on defense by almost 5 percent — meaning­ there have now been three consecutive years of growth since 2014.” Still, the report showed that in 2017 only five NATO members had met the 2 percent target – the U.S., U.K., Greece, Poland and Estonia.

– CNBC's Holly Ellyatt contributed to this report.