BRUSSELS, Belgium (AFP) — President Donald Trump told NATO allies to “reimburse” the United States for defence costs as he landed for the alliance’s most difficult summit in years.

Trump touched down in Brussels aboard Air Force One hot on the heels of a stern warning from the EU’s top official that America should “appreciate” its allies.

The leaders of the other 28 members of the NATO alliance are hoping for a show of unity despite stark transatlantic tensions on a host of issues, most notably Trump’s repeated invective against Europe over defence spending.

“Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2 per cent (which is low), but are also delinquent for many years in payments that have not been made,” he tweeted while his plane was in the air. “Will they reimburse the US?”

Trump had said before leaving Washington that his meeting in Helsinki with Russian President Vladimir Putin next Monday “may be the easiest” part of his upcoming European tour, which also includes a trip to Britain where the Government is in crisis over Brexit.

“Frankly, Putin may be the easiest of them all. Who would think?” he told reporters.

He vowed not to be “taken advantage” of by the European Union, which he accuses of freeloading by relying on the United States for its defence while blocking US imports into the bloc, the world’s biggest market.

“It’s certainly going to be an interesting time with NATO,” Trump told journalists.

The meeting of Western leaders in Brussels has the potential to turn into another public bust-up, following a divisive and bad-tempered summit of G7 nations in Canada in June.

Trump has ramped up his rhetoric ahead of the talks — including three separate tirades on Twitter on Monday and Tuesday — making the summit one of the most difficult in years for the military alliance that has underpinned European security since World War II.

European officials have expressed hopes that member states can paper over their differences, but European Union President Donald Tusk delivered a blunt message to the US leader on Tuesday.

“Dear America, appreciate your allies, after all you don’t have that many,” Tusk said, before reminding Trump that European troops had fought alongside Americans in Afghanistan following the September 11, 2001 attacks on the United States.

“Please remember this tomorrow when we meet at the NATO summit, but above all when you meet President Putin in Helsinki. It is always worth knowing who is your strategic friend and who is your strategic problem,” he said.

Trump will meet the Russian leader in the Finnish capital on July 16 for their first summit amid an ongoing investigation in the US into possible Trump campaign collusion with Russia.

Many European diplomats fear a rerun of a sequence in June when Trump clashed with his Western allies at the G7 summit, calling Canadian Prime Minister Justin Trudeau “dishonest and weak”, then praised North Korean dictator Kim Jong-Un as “very talented” at a summit afterwards.

“He chews out the allies then embraces the adversary,” one diplomat told AFP recently.

Trump set the stage for clashes at the NATO summit by writing to around a dozen allies to berate them for lagging on a 2014 pledge to try to spend two per cent of GDP on defence by 2024.

Speaking to a cheering crowd at a rally this week, Trump told them that the US would no longer be “the schmucks paying for the whole thing”.