President Trump on Tuesday embarked on a weeklong trip across Europe – starting with a stop in Belgium for the NATO summit.

During his trip, Trump will sit down with Russian President Vladimir Putin and head to the U.K. where Prime Minister Theresa May’s government is facing controversy over her plans for exiting the European Union.

But it’s the North Atlantic Treaty Organization (NATO) – the two-day summit and organization – that have been the subject of Trump’s Twitter feed in recent weeks.

“NATO countries must pay MORE, the United States must pay LESS,” Trump tweeted on July 10, just hours before he took off for Europe. “Very Unfair!”

NATO is a group of 29 countries that have a political and military alliance. Aside from the U.S., the group consists of Belgium, Canada, France, Italy, the U.K., Germany and Spain, among others.

Trump has been regularly vocal of NATO, especially in the days leading up to his trip. As he departed the White House en route to Brussels Tuesday, Trump said of the pact, “Frankly it helps them a lot more than it helps us.”

He’s been especially critical of NATO countries failing to fulfill their goal of spending 2 percent of their gross domestic products on defense by 2024. During his presidential campaign, he suggested he might only come to the defense of NATO countries that have fulfilled that obligation.

TRUMP SAYS PUTIN SUMMIT COULD BE ‘EASIEST’ STOP OF EUROPEAN TOUR, AMID NATO TENSIONS

Defense spending among the nations did increase overall in 2017, but the U.S. still largely outspent the other members, according to Fox Business. Aside from the U.S., only four other countries meet the 2 percent threshold: Estonia, Greece, Poland and the U.K.

The president’s comments haven’t gone unnoticed. Earlier Tuesday, European Council President Donald Tusk lashed out at Trump, saying, “Appreciate your allies, after all you don’t have that many.”

Ahead of the trip, Sen. John McCain, R-Ariz., reaffirmed an “overwhelming bipartisan support in the Congress for the transatlantic alliance.” McCain, chairman of the Senate Armed Services Committee, said he hopes Trump will “use the opportunity presented by the NATO summit to reaffirm the values of a transatlantic alliance that continues to have a real and profound influence on global security.”

“As we face the most complex and dangerous security environment since the end of the Cold War, we must not forget that America is safer and more secure because we work with and through our allies. Throughout the past seven decades, the United States and its NATO allies have served together, fought together, and sacrificed together for a vision of the world based on freedom, democracy, human rights and rule of law,” McCain said. “Our enduring alliance stands as an important safeguard in preserving this world order – and it is essential to securing our national interests.”

TRUMP ‘MAKING AN IMPACT’ ON NATO AS ALLIES RAMP UP DEFENSE SPENDING, AMBASSADOR SAYS

Trump’s attitude toward NATO has concerned many and left some speculating whether Trump would alter America’s commitment to the group.

“Although the Atlantic alliance has weathered many crises over its lifetime, I now am concerned that the alliance will not survive Donald Trump,” Elizabeth Sherwood-Randall, who served as senior director for European affairs on the National Security Council under President Barack Obama, told The Associated Press.

“Because he isn't arguing with allies about policy. He's questioning the fundamental value of NATO to the United States. This antagonistic approach is generating an unprecedented debate in Europe and in Canada about whether the United States should be treated as friend or foe,” she said.

Tomas Valasek, a former Slovak ambassador to NATO who now runs Carnegie Europe, a Brussels think tank, told The Washington Post, “Deterrence has already been broken.”

“The biggest of the allies doesn’t just have a disagreement with us, but he actually seems willing to walk away,” he said.

The Associated Press contributed to this report.