President Trump said Tuesday that an upcoming summit with Russian President Vladi­mir Putin might be easier than a gathering with NATO allies, as he left the White House for a week of high-profile diplomacy in Europe.

In tweets and comments to reporters before departing, Trump took fresh aim at other NATO members for not meeting targets for defense spending, saying the arrangement was unfair to U.S. taxpayers.

His trip to Europe will also include a meeting in Britain with Prime Minister Theresa May, whose government is in disarray after a pair of key resignations, and a weekend of golf in Scotland.

“So I have NATO, I have the U.K. — which is somewhat in turmoil,” Trump told reporters. “And I have Putin. Frankly, Putin may be the easiest of them all. Who would think? Who would think?”

Asked if he considers Putin a friend or a foe, Trump said: “I really can’t say right now. As far as I’m concerned, he’s 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 added. “I’ve said that many times for many years. So we’ll see. We’re meeting with Vladimir Putin on Monday. We’ll see how that goes.”

Trump also declined to say whether May should stay in power.

“Well, that’s up to the people,” he said. “I get along with her very well. I have a very good relationship. That’s certainly up to the people, not up to me.”

Trump also offered kind words for Boris Johnson, the former foreign secretary who is a frontman for Britain’s campaign to leave the European Union and one of the recent departures from May’s government.

“Boris Johnson’s a friend of mine,” Trump said. “He’s been very, very nice to me. Very supportive. And maybe we’ll speak to him when I get over there.”

Trump’s comments came in advance of a week-long trip that has already riled many leaders within the North Atlantic Treaty Organization.

In a Fox News interview later Tuesday, Vice President Pence appeared to downplay Trump's remarks on Russia.

“The president made it clear that we have competing interests with Russia,” Pence said in the interview.

He cited Syria and Ukraine as potential areas of cooperation and maintained that Trump “believes in sitting down with leaders and determining whether there’s a way that we can make progress in the relationship.”

“I think he goes into this summit very much in that spirit,” Pence said.

Days after their meeting in Brussels, Trump plans to sit down with Putin for their first one-on-one summit, a meeting in Helsinki, that could be more cordial, further straining the long-standing military alliance.

“Getting ready to leave for Europe. First meeting — NATO,” Trump wrote in early morning tweets. “The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer.”

In a second tweet, the president wrote: “NATO countries must pay MORE, the United States must pay LESS. Very Unfair!”

Hours later, aboard Air Force One en route to Brussels, Trump returned to the subject, with another tweet.

“Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made,” he wrote. “Will they reimburse the U.S.?”

Trump has long been spoiling for a fight on the issue of defense spending, latching onto the notion that each NATO country should be paying at least 2 percent of its gross domestic product toward its military.

“I’m going to tell NATO — you got to start paying your bills,” he told a rowdy rally in Montana last week. “The United States is not going to take care of everything. We are the schmucks that are paying for the whole thing.”

Last month, he sent letters to leaders whose countries are not living up to their NATO defense spending pledges, warning that the United States might cut them off if they don’t pour more money into their militaries. But NATO members, including Germany, argue they have boosted contributions as part a pledge to kick in at least 2 percent of annual economic output by the middle of the next decade.

Trump also has questioned why the United States should run a trade deficit with nations it is spending money to protect, suggesting he could use security guarantees as a bargaining chip in trade talks.

Trump’s tweet on Tuesday echoed another one from Monday, in which he singled out Germany for not meeting defense spending targets.

Before departing the White House shortly after 7 a.m., Trump also fired off another tweet claiming credit for an economy that is “ROARING” and a Supreme Court nomination that he said has received “GREAT REVIEWS.”

He also noted a poll of his job approval among fellow Republicans, writing: “Wow!”

Michael Birnbaum in Brussels and Felicia Sonmez in Washington contributed to this report.