As Donald Trump arrives in Europe for what is expected to be a fractious encounter at a two-day Nato summit, the President has been pointedly reminded to “appreciate your allies”.

Mr Trump hit out at the European Union on trade and at his Nato allies for failing to spend enough on defence shortly before boarding Airforce One with wife Melania for what will be an extended European tour.

“NATO has not treated us fairly … We pay far too much and they pay far too little,” Mr Trump said as he left the White House on Tuesday night.

But while the refrain was not new, the response from European Council President Donald Tusk was of frankness rarely seen in Europe’s dealings with Mr Trump.

“Dear America, appreciate your allies, after all you don’t have that many,” Mr Tusk posted on Twitter, adding that the EU spent more than Russia on defence, and as much as China.

He said the US did not and would not have a better ally than the EU, reminding the President that it was European troops who fought and died in Afghanistan after the September 11 attacks on the US in 2001.

Dear @realDonaldTrump. US doesn’t have and won’t have a better ally than EU. We spend on defense much more than Russia and as much as China. I hope you have no doubt this is an investment in our security, which cannot be said with confidence about Russian & Chinese spending 🙂

— Charles Michel (@eucopresident) July 10, 2018

The NATO alliance is looking stronger militarily than at any time since the Cold War and with plans to expand further to curb Russian power.

But while NATO has much to be triumphalist about as it stages its first biennial summit later Wednesday at its new billion dollar headquarters in Brussels, many summit leaders paradoxically appear anxious as they face the alliance’s de-facto leader, Mr Trump, who brings with him tough talk on defence spending.

Accusations that Russia is trying to destabilise the West with cyber attacks and covert action have laid the ground for the North Atlantic Treaty Organisation’s biggest expansion for decades, backed by a surge in US defence spending in Europe.

The meeting brings together more than 40 heads of government including the 29 allies and non-member partners from Finland to Afghanistan, underlining the organisation’s reach.

NATO is also set to expand again, inviting Macedonia to start accession talks and defying Russian warnings against enlargement of the US-led bloc deeper into the Balkans.

But Mr Trump’s comments about many allied governments not paying their way in the alliance in exchange for the US umbrella has thrown many summit leaders on the back foot.

Carefully-choreographed sessions and the leaders’ dinner in a Brussels museum on Wednesday night local time are unlikely to mollify Mr Trump, NATO diplomats say, as they found out to their dismay in May last year at a special dinner to welcome the President.

Back then, Mr Trump spoke his mind, ignoring decorum and warning NATO allies that they owed “massive sums” and had to do more to stop terrorism.

No one knows for sure what Mr Trump will say at NATO and diplomats also worry that European leaders might react, making a tense situation worse.

-With agencies