WASHINGTON (AP) — 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.

European Council President Donald Tusk said on Tuesday in a message to Trump that “it is always worth knowing who is your strategic friend and who is your strategic problem.” Tusk recalled that the Europeans are spending more than Russia and as much as China on defense. NATO estimates that 15 members, or just over half, will meet the benchmark by 2024 based on current trends.

“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,” Trump tweeted Tuesday morning, adding: “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)!”

On Monday he’d tweeted the situation was “not fair, nor is it acceptable,” and insisted that NATO benefits Europe “far more than it does the U.S.”

He added: “NATO countries must pay MORE, the United States must pay LESS. Very Unfair!”

Trump, who has compared the sentiment that underpinned 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.

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By JILL COLVIN and JONATHAN LEMIRE , Associated Press

Lemire reported from New York.