President Donald Trump's four-nation European tour has allies fretting over the risk of damage he could do to the decades-old NATO alliance. They're also worried about his potential embrace of Russia's Vladimir Putin during a summit in Helsinki.

The trip that begins Tuesday in Brussels will also take Trump to London, where Prime Minister Theresa May's government is in turmoil over her plans for exiting the European Union.

Trump has been pressing NATO countries 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 nations that fulfilled their obligation. He continues to criticize NATO countries for not paying their fair share.

NATO estimates that 15 members, or just over half, will meet the benchmark by 2024 based on current trends.

"The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable," Trump tweeted Monday, insisting that NATO benefits Europe "far more than it does the U.S."

"On top of this the European Union has a Trade Surplus of $151 Million with the U.S., with big Trade Barriers on U.S. goods. NO!" he protested.

Trump, who has compared the Brexit vote to leave the EU to his own election, will be making his maiden presidential trip to Britain at a fraught time for May. Foreign Secretary Boris Johnson and Brexit Secretary David Davis resigned within hours of each other in protest of her plan.

Trump's visit is expected to attract large protests in London and elsewhere in Britain.

Trump's weeklong trip to Europe will continue with a stop in Scotland before ending with a sit-down in Helsinki with Putin, whose country the U.S. intelligence community has concluded interfered in the 2016 election to help Trump win.

The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied the allegations of election meddling, in spite of evidence to the contrary.