President Donald Trump criticized NATO on Tuesday in a pair of tweets ahead of his seven-day European trip this week, accusing U.S. allies of exploiting America on defense spending.

"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)!" the president wrote in a tweet, adding in a later tweet, "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"

Trump will attend the NATO summit in Brussels on Wednesday and Thursday, then travel to England, where he will meet Prime Minister Theresa May and Queen Elizabeth II before heading to Scotland. He will cap it off with a summit on Monday with Russian President Vladimir Putin in Helsinki.

Trump has previously attacked NATO, the military alliance of 29 countries across North America and Europe formed in the aftermath of World War II, for shortchanging the U.S. and vowed to upend the crucial relationship if other member countries did not contribute more.

Donald Tusk, president of the European Council, responded to Trump's claims on Tuesday, criticizing the president for attacking its allies.

"America does not have, and will not have a better ally than Europe," Tusk said at a joint E.U.-NATO declaration Tuesday. “Dear America, appreciate your allies, after all you don’t have all that many."

Tusk noted that Europe has promised to increase common defense spending, but noted that the E.U. contributes more than Russia and as much as China.

Trump’s visit is expected to draw large protests in the U.K. Tens of thousands of demonstrators are expected to march in London, Scotland and elsewhere during his trip amid growing trade tensions between Europe and the U.S. and objection to the president's immigration policies.

Before departing for his trip on Marine One on Tuesday, Trump continued to criticize the E.U. for its NATO contributions and reacted to Tusk's comments about the scarcity of America's allies.

"Well, we do have a lot of allies, but we cannot be taken advantage of, we are being taken advantage of by the European Union," Trump said, adding that NATO "helps them a lot more than it helps us."

He also dismissed criticism of him meeting with Putin, saying that it was in America's interest and that he did not see Russia as an adversary, but as a "competitor."

"I think that getting along with Russia, getting along with China, getting along with others is a good thing, not a bad thing," he said.

Trump quipped that his meeting with Putin "would be the easiest of them all," adding that Britain is in "turmoil" following the resignations of Foreign Secretary Boris Johnson and Brexit Secretary David Davis.

Trump, who called Johnson a "friend," added it would be "up to the people" if May is to remain prime minister, but said they have a good relationship.