European Council President Donald Tusk has told US President Donald Trump to stop berating NATO allies over military investment levels ahead of what is expected to be a fraught alliance summit.
Before taking off for Brussels, Mr Trump again chided fellow NATO members for not contributing enough to the alliance while maintaining a trade surplus with the United States.
Accusing the president of criticising Europe "almost daily", Mr Tusk said the EU spent more than Russia on defence and as much as China.
He said that money helped boost US security, as the United States regards Russia and China as threats.
"Dear America, appreciate your allies, after all you don't have that many," Mr Tusk, a former Polish prime minister, said.
Mr Tusk also said that the US did not and would not have a better ally than the EU, reminding the president that it was European troops who had fought and died in Afghanistan after the 11 September 2001 attacks on the US.
As European Council president, Mr Tusk serves as the chairman when EU leaders meet at summits.
After attending the Nato summit on Wednesday, President Trump will spend four days in the UK, which is in the grip of a political crisis, before meeting Russia's Vladimir Putin in Helsinki on Monday.
While Mr Trump is looking to improve relations with Russia, there has been alarm at the deteriorating climate with his allies in Europe and what he might agree to with President Putin.
Some have expressed fears for the future of Nato itself, and German Chancellor Angela Merkel has indicated that Europe may no longer be able to rely on its US ally.
Mr Trump told reporters as he boarded Air Force One: "So I have Nato, I have the UK which is in somewhat turmoil, and I have Putin. Frankly, Putin may be the easiest of them all. Who would think?"
The EU Council leader said pointedly that when Mr Trump did meet Mr Putin on 16 July "it is always worth knowing who is your strategic friend and who is your strategic problem."
Before he left Andrews Air Force Base, President Trump reiterated his demand for the US to pay less and other members of the Western military alliance to pay more, and he linked the issue to the EU's trade surplus with the US.
British Prime Minister Theresa May's spokesman said he hoped Mr Trump would have a positive image of Britain following his visit this week after Mr Trump's remarks on the UK's "turmoil".
On Nato, President Trump's main objection is that a number of member states have not increased their defence budgets to the target of 2 percent of economic output.
Nato countries have agreed to raise military spending by 2024, although Germany and Spain are unlikely to meet the target.
Nato on Tuesday released new estimates for allied defence spending in 2018, showing that Latvia and Lithuania are set to meet the 2 percent target. Of the 29 allies, only the United States, Britain, Estonia, Greece and Poland met the goal in 2017. Romania is also on course to hit it in 2018, NATO said.
Many US presidents have urged European governments to spend more on their militaries. But Mr Trump has intensified the demands to such an extent that allies worry it could damage Nato morale and play into the hands of Russian President Vladimir Putin, who they accuse of trying to destabilise the West.
Nato secretary-general Jens Stoltenberg stressed that all allies were increasing defence spending in real terms because economies in Europe were growing. He also said Washington strengthens its security and global reach through NATO.
European officials say while US defence spending makes up 70 percent of combined allied governments' military budgets, just 15 percent of US expenditure is spent in Europe on NATO-related defence. Washington pays about 22 percent of the running cost of NATO, including the headquarters and commonly-funded equipment such as AWACS surveillance planes.
-Reuters / BBC