European Union President Donald Tusk on Tuesday delivered a strongly worded statement directed at President Donald Trump amid his ongoing criticism of NATO: "Appreciate your allies. After all, you don't have that many."

Tusk's remarks come after sustained criticism of NATO by Trump in relation to how it's funded.

"Money is important, but genuine solidarity is even more important," Tusk said.

European Union President Donald Tusk on Tuesday delivered a strongly worded statement directed at President Donald Trump amid his ongoing criticism of NATO: "Appreciate your allies. After all, you don't have that many."

As Trump was set to travel to Brussels for a NATO summit, Tusk said, "I would like to address President Trump directly, who for a long time now has been criticizing Europe almost daily for, in his view, insufficient contributions to the common defence capabilities, and for living off the US."

"Dear President Trump: America does not have, and will not have a better ally than Europe," Tusk added. "Money is important, but genuine solidarity is even more important."

Tusk outlined the ways in which Europe has stood by the US in recent years, including after the 9/11 terror attacks. He also noted that Europeans spend about as much on defense as China and more than Russia.

The EU president said he hoped Trump would remember Europe's strong relationship with the US as he meets with Russian President Vladimir Putin in Helsinki in the coming days.

Tusk's remarks come after sustained criticism of NATO by Trump.

On Tuesday, Trump tweeted, "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. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"

Trump's statements on the way NATO operates and is funded have often been misleading.

Trump's suggestion the US provides protection for other NATO member states glosses over the founding premise of the alliance: collective defense. In this sense, all of the member states provide protection for one another. Furthermore, all member states contribute to NATO's overall budget based on their national income, with the US providing about 22% of the budget.

In other words, the US government's contribution to NATO is proportional, and many have criticized Trump for claiming Europe is taking advantage of the US in this regard.

Trump's attacks on NATO come as many European countries express concern about Russia's aggression in the region, especially in the wake of the annexation of Crimea in 2014. In this context, Tusk and others seem concerned Putin will take advantage of the fact that Trump is not standing by key US allies.

Referencing Trump's impending summit with Putin, Tusk on Tuesday said, "It is always worth knowing: who is your strategic friend? And who is your strategic problem?"

Meanwhile, as Trump spoke with reporters before heading off to Europe, he said his meeting with Putin could be the "easiest" aspect of his trip.