President of the European Council Donald Tusk has accused US President Donald Trump of being averse to a "strong and united" Europe, and also warned against the emergence of a "brownshirt" nationalist front in EU elections next year.
His comments came as Mr Trump, in France for WWI centenary commemorations, criticised French President Emmanuel Macron over his proposals for a European army.
"For the first time in history we have an American administration that is, to put it mildly, not enthusiastic about a united and strong Europe," Mr Tusk said in a speech in his native Poland.
"I'm speaking about facts, not propaganda," said the EU chief on the eve of celebrations to mark Poland's 100th anniversary of independence.
The EU Council president previously voiced fears that Mr Trump was seeking to tear down the post World War II order, with the US leader once calling the EU a foe in trade.
In an interview published by the Polish newspaper Gazeta Wyborcza, Mr Tusk also said he believed Mr Trump wanted a world with "the United States on one side and on the other, a disparate collection of other countries".
Mr Trump on Friday posted a tweet berating Mr Macron's calls for a European army, but the French president on Saturday sought to ease the row, hailing the "great solidarity" between the two countries.
Looking ahead to the European Parliament elections in May, Mr Tusk warned against the emergency of a nationalist front opposed to the EU itself.
"It cannot be ruled out that there will be two streams represented: one in the colours of the brownshirts - anti European and focused on nationalism - and the second which wants to push as much as possible for EU integration," said Mr Tusk.
The former Polish prime minister also spoke of rising anti-European sentiment in several capitals, saying some forces wanted "conflict rather than cooperation, disintegration rather than integration".
Without naming names, Mr Tusk criticised Polish leaders for their stance on Europe, after this week warning that the eurosceptic ruling party's policies carried the risk of a "Polexit".
"Those who are opposed to a strong Polish presence in Europe are de facto against our independence," he declared.
Mr Tusk's comments were a response to earlier statements by Donald Trump, who said that the suggestion for a joint European army was "very insulting".
As Mr Trump's plane landed in Paris for the WWI commemoration ceremonies, he took to Twitter to say: "President Macron of France has just suggested that Europe build its own military in order to protect itself from the US, China and Russia.
"Very insulting, but perhaps Europe should first pay its fair share of NATO, which the US subsidises greatly!"
President Macron of France has just suggested that Europe build its own military in order to protect itself from the U.S., China and Russia. Very insulting, but perhaps Europe should first pay its fair share of NATO, which the U.S. subsidizes greatly!— Donald J. Trump (@realDonaldTrump) November 9, 2018
But this morning, he played down the row over defence spending.
"We want a strong Europe, it's very important to us and whichever way we can do it the best and more efficient would be something we both want," Mr Trump said after being greeted by Mr Macron.
Asked what he meant when he tweeted that he felt insulted by Mr Macron's comments that Europe should reduce its dependence on the United States for security, Mr Trump said: "We want to help Europe but it has to be fair.
"Right now the burden sharing has been largely on the United States."
Mr Macron said he shared Mr Trump's view that Europe needed to finance a greater share of the NATO military alliance's costs.
"That's why I do believe my proposals for European defence are fully consistent with that," he said, speaking in English.
Earlier, Mr Macron, who is an ardent advocate of closer European integration, said a joint European Union military force was needed to wean Europe off American might, not least after Mr Trump announced he was pulling out of a Cold War-era nuclear treaty.
The French President told France’s Europe 1 radio: "We have to protect ourselves with respect to China, Russia and even the United States".
He is specifically pushing for the creation of a nine-country European force, independent from NATO, that could rapidly mount a joint military operation, evacuate civilians from a war zone, or provide aid after a natural disaster.
Mr Trump's reaction is likely to add to strains in transatlantic relations, which will be under the spotlight this weekend as 70 world leaders, including Germany’s Chancellor, Angela Merkel, and the Russian President, Vladimir Putin, gather in Paris to mark the centenary of the end of World War One.