Trump has ramped up criticism of European defence spending in recent days, stressing how many EU members are failing to meet agreed targets for investments, which has raised fears the NATO summit could lead to another bust-up in the transatlantic alliance.
Speaking at a signing ceremony for a new EU-NATO cooperation agreement, Tusk used the opportunity to send a personal message to Trump, who is due to arrive in the Belgian capital later today.
"The US doesn't have and won't have a better ally than the EU. We spend on defence much more than Russia and as much as China," he added. "Dear America, appreciate your allies, after all you don't have that many."
He concluded with a reminder that European troops had fought and lost their lives 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 (Vladimir) Putin in Helsinki. It is always worth knowing who is your strategic friend and who is your strategic problem," he said.
"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)!" he wrote on Twitter.
"NATO countries must pay MORE, the United States must pay LESS. Very Unfair!" he said in a second message.
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