United States President Donald Trump laid the groundwork for a fraught Nato summit on Tuesday as he set off on a week-long Europe trip.

The US president took to lambasting allies on social media and claiming his meeting with Russian President Vladimir Putin would be the “easiest” leg of the diplomatic tour.

As he left the White House for Brussels, Mr Trump said the alliance’s members were contributing too little and that he would have no problems in his meeting with the Russian leader, heightening fears in Brussels about a further tear in the already damaged transatlantic alliance.

Yet, for all of the acrimony around his treatment of European allies, Mr Trump appeared to care little as he set off for the continent.

Speaking to reporters on the South Lawn of the White House, he said that Britain was in “turmoil” after two high-profile resignations over Brexit and that it was “certainly going to be an interesting time with Nato”.

He said that the decades-long security pact “helps them a lot more than it helps us”.

And Mr Putin? “Frankly, [he] may be the easiest of them all,” he said. “Who would think?”

Nato did not respond to a request for comment about Mr Trump’s remarks, and alliance officials have been careful about publicly and privately criticising the president ahead of his visit.

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Read more from Jack Moore in Brussels:

Nato's glistening new headquarters is home to an alliance with old problems

Europe aims to avoid disaster as Nato prepares for a Trump tornado

Nato Summit: The key issues on the table for alliance

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As Mr Trump carried out his verbal assault on Europe, Nato Secretary-General Jens Stoltenberg instead looked to appease Mr Trump, thanking him for his “leadership” in pushing for greater defence spending even as the president attacked allies ahead of the summit.

Speaking at the alliance’s new headquarters in Brussels before Mr Trump’s comments on Tuesday, the Nato leader emphasised a message of unity and talked up efforts by Canada and European members to boost their defence – a response to US complaints about lopsided contributions in the 29-member alliance.

“I would like to thank President Trump for his leadership on defence spending. It is clearly having an impact,” he said.

“My message is that now European allies are stepping up, Canada is stepping up, we are doing more together. I am absolutely certain that this summit will show that we are able to deliver on defence despite disagreements on other issues.”

Mr Trump has threatened that he would only protect countries who met the 2 per cent defence spending obligation the alliance states agreed to meet by 2024.

Looking for signs that the president remained committed to the alliance, Mr Stoltenberg pointed to a rise in the US presence in Europe as well as a spending increase of 40 per cent on defence in Europe since the US president’s inauguration.

“Actions speak louder than words,” he said.

But Mr Trump, who will arrive in the Belgian capital on Tuesday, pinged off a series of combative tweets taking aim at European spending.

“Getting ready to leave for Europe. First meeting – Nato. The US is spending many times more than any other country in order to protect them. Not fair to the US taxpayer,” he wrote.

In a second tweet, he said: “Nato countries must pay MORE, the United States must pay LESS. Very Unfair!”

In an earlier tweet on Monday, he said the situation within Nato was “not fair, nor is it acceptable”.

NATO countries must pay MORE, the United States must pay LESS. Very Unfair! — Donald J. Trump (@realDonaldTrump) July 10, 2018

In response to American complaints, the Nato chief said he expected three more countries, without specifying which members, to reach the 2 per cent threshold of defence spending per GDP this year.

Nato estimates suggest that European members and Canada will add US$266 billion (Dh977bn) in defence spending between now and 2024, the target year for all members to reach the threshold, according to Mr Stoltenberg.

The alliance says that 15 members, or just over half, will meet the benchmark by that year, based on current trends.

“Nations were cutting defence spending by billions of dollars, now they are adding billions of dollars,” he said.

Experts say that while Mr Trump can claim some credit for a change in spending at Nato, his actions are still damaging for Washington in the wider world.

“The President of the United States is irrational and unconvinced of the value of the transatlantic alliance,” Marcel Dirsus, political scientist at the University of Kiel, says. “He is unconcerned with reality and fond of Putin. That's bound to lead Europeans to spend more on defence, but it doesn't further the American national interest.”

European allies are so concerned about the Trump effect that they are making moves to strengthen their voices as one and combat any negative impact he may have on the 69-year-old alliance.

“Several European governments have concluded that they would have to build a European caucus inside Nato,” said Josef Janning, head of the Berlin office at the European Council on Foreign Relations. “Europe needs more of a single voice within the alliance.”

After the summit, Mr Trump will travel to Britain for a state visit but, more concerning for Brussels, he will host his first one-on-one summit with Mr Putin in Helsinki on July 16.

The Brussels elite fears that he will embrace Mr Putin, despite the Russian annexation of Crimea and incursions into Ukraine. The Russian leader denies meddling in the US presidential election and other European votes. Mr Trump’s campaign team remains under investigation for possible collusion with Moscow to secure his victory over Hillary Clinton in the 2016 US presidential election.

At an earlier press conference with Mr Stoltenberg on Tuesday, European Council President Donald Tusk, a longtime critic of the president, sent a warning to Mr Trump that is likely to have been ill-received in Washington.

"It is always worth knowing who is your strategic friend and who is your strategic problem," he said, in comments that appeared to allude to Russia.