European Council President Donald Tusk has told the US President that he should know "who is your strategic friend and who is your strategic problem" as the pair clashed on Twitter.
Mr Tusk appealed to President Donald Trump to remember who his allies are on the eve of the NATO summit in Brussels.
In a blunt message to the US leader, he said: "it is always worth knowing who is your strategic friend and who is your strategic problem," while signing a joint EU-NATO declaration with alliance Secretary-General Jens Stoltenberg.
The US president bit back on Twitter, writing: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"
NATO countries must pay MORE, the United States must pay LESS. Very Unfair!
— Donald J. Trump (@realDonaldTrump) July 10, 2018
A few moments earlier, Mr Trump tweeted he was preparing for his journey overseas 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)!"
Mr Tusk responded to the US president on Twitter, claiming that the US won't have "a better ally than EU."
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 :-)
— 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 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 :-)".
During his personal message to the US leader at the signing ceremony, Mr Tusk added: "Dear America, appreciate your allies, after all you don't have that many."
Mr Trump has ramped up criticism of European defence spending over the past week.
He has stressed how many EU members are failing to meet agreed targets for investments, raising fears the NATO summit could spark another rift in the transatlantic alliance.
