Donald Tusk has told Donald Trump he should know "who is his strategic friend and who is his strategic problem" as the pair clashed on Twitter.
The European Council President appealed to Mr Trump to remember who his allies are as he signed a joint declaration alongside NATO Secretary-General Jens Stoltenberg.
It comes on the eve of the NATO summit in Brussels.
The US president bit back on Twitter, writing: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"
A few moments earlier, Mr Trump tweeted he was preparing for his journey to Europe and took aim at NATO and the EU.
Getting ready to leave for Europe. First meeting – NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!
— Donald J. Trump (@realDonaldTrump) July 10, 2018
NATO countries must pay MORE, the United States must pay LESS. Very Unfair!
— Donald J. Trump (@realDonaldTrump) July 10, 2018
"Getting ready to leave for Europe. First meeting – NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"
Mr Tusk responded to the US president on Twitter, claiming that the US "won't have a better ally than EU".
Dear @realDonaldTrump. US doesnt have and wont 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
— Donald Tusk (@eucopresident) July 10, 2018
The former Polish prime minister said: "Dear @realDonaldTrump. US doesn't have and won't have a better ally than EU.
"We spend on defence 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 :-)"
During the signing, Mr Tusk added: "Dear America, appreciate your allies, after all you don't have that many."
He offered a reminder that European troops had fought and lost their lives alongside Americans following the September 11 attacks on the US in 2001.
"Please remember this tomorrow when we meet at the NATO summit, but above all when you meet President (Vladimir) Putin in Helsinki," he said.
Mr Trump has ramped up criticism of European defence spending over the past week.
He has stressed that many EU members are failing to meet agreed targets for investments, raising fears the NATO summit could spark another rift in the transatlantic alliance.
The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable. While these countries have been increasing their contributions since I took office, they must do much more. Germany is at 1%, the U.S. is at 4%, and NATO benefits…….
— Donald J. Trump (@realDonaldTrump) July 9, 2018
…Europe far more than it does the U.S. By some accounts, the U.S. is paying for 90% of NATO, with many countries nowhere close to their 2% commitment. On top of this the European Union has a Trade Surplus of $151 Million with the U.S., with big Trade Barriers on U.S. goods. NO!
— Donald J. Trump (@realDonaldTrump) July 9, 2018
On Monday, he tweeted: "The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable. While these countries have been increasing their contributions since I took office, they must do much more."
Mr Trump is set to meet Mr Putin, the Russian president, in the Finnish capital on 16 July.
More from Donald Trump
Relations between the US and Russia have become strained over allegations of Russian interference in the 2016 US presidential election, as well as Moscow's support for Syrian President Bashar al Assad and its involvement in the Ukraine conflict.
Mr Trump and Mr Putin will discuss relations between their countries and "a range of national security issues" during the long-awaited meeting, said the White House.
Donald Tusk has told Donald Trump he should know "who is his strategic friend and who is his strategic problem" as the pair clashed on Twitter.
The European Council President appealed to Mr Trump to remember who his allies are as he signed a joint declaration alongside NATO Secretary-General Jens Stoltenberg.
It comes on the eve of the NATO summit in Brussels.
The US president bit back on Twitter, writing: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"
A few moments earlier, Mr Trump tweeted he was preparing for his journey to Europe and took aim at NATO and the EU.
Getting ready to leave for Europe. First meeting – NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!
— Donald J. Trump (@realDonaldTrump) July 10, 2018
NATO countries must pay MORE, the United States must pay LESS. Very Unfair!
— Donald J. Trump (@realDonaldTrump) July 10, 2018
"Getting ready to leave for Europe. First meeting – NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"
Mr Tusk responded to the US president on Twitter, claiming that the US "won't have a better ally than EU".
Dear @realDonaldTrump. US doesnt have and wont 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
— Donald Tusk (@eucopresident) July 10, 2018
The former Polish prime minister said: "Dear @realDonaldTrump. US doesn't have and won't have a better ally than EU.
"We spend on defence 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 :-)"
During the signing, Mr Tusk added: "Dear America, appreciate your allies, after all you don't have that many."
He offered a reminder that European troops had fought and lost their lives alongside Americans following the September 11 attacks on the US in 2001.
"Please remember this tomorrow when we meet at the NATO summit, but above all when you meet President (Vladimir) Putin in Helsinki," he said.
Mr Trump has ramped up criticism of European defence spending over the past week.
He has stressed that many EU members are failing to meet agreed targets for investments, raising fears the NATO summit could spark another rift in the transatlantic alliance.
The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable. While these countries have been increasing their contributions since I took office, they must do much more. Germany is at 1%, the U.S. is at 4%, and NATO benefits…….
— Donald J. Trump (@realDonaldTrump) July 9, 2018
…Europe far more than it does the U.S. By some accounts, the U.S. is paying for 90% of NATO, with many countries nowhere close to their 2% commitment. On top of this the European Union has a Trade Surplus of $151 Million with the U.S., with big Trade Barriers on U.S. goods. NO!
— Donald J. Trump (@realDonaldTrump) July 9, 2018
On Monday, he tweeted: "The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable. While these countries have been increasing their contributions since I took office, they must do much more."
Mr Trump is set to meet Mr Putin, the Russian president, in the Finnish capital on 16 July.
More from Donald Trump
Relations between the US and Russia have become strained over allegations of Russian interference in the 2016 US presidential election, as well as Moscow's support for Syrian President Bashar al Assad and its involvement in the Ukraine conflict.
Mr Trump and Mr Putin will discuss relations between their countries and "a range of national security issues" during the long-awaited meeting, said the White House.
