Tusk's letter comes as European leaders from Germany to Iceland to Britain condemn Trump's immigration ban, and a day after Guy Verhofstadt, the EU's lead negotiator for Brexit talks, similarly characterized the US president as a threat to the union on par with terrorism.
British Prime Minister Theresa May, about to lead her country out of the European Union, briefed peers on her visit to Washington last week and assured them Trump was committed to cooperating in their defence - just as Britain would also be after Brexit.
Even before the summit, EU Council President Donald Tusk of Poland spoke of "worrisome declarations by the new American administration", and characterized the United States as an external "threat" along with "Russia's aggressive policy", an "increasingly, let us call it, assertive China" and the "wars, terror and anarchy in the Middle East and in Africa".
The summit wrapped up with leaders - minus Britain's May - discussing a major declaration on the EU's future that they plan to unveil at a summit in Rome in March marking the 60th anniversary of the bloc's founding treaty. I think it is a balancing job the Prime Minister must make.
He toned down his comments on Friday, saying that USA ties were the EU's "highest political priority", but still stressed that Europeans would have to stand on their own feet now.
She said that the alternative of "division and confrontation" would "embolden" those who wished to do the European Union harm. "What matters is European Union solidarity". "It is up to Europe to decide how many members it should have and how it should live". Before arriving at the Valletta summit, Orban said that the U.S. has the right to decide its own border control policy, and that he is puzzled at the "neurotic European reactions" over the travel ban.
"We must not imagine some sort of external protection", he said.
"It can not be accepted that there is, through a certain number of statements by the president of the United States, pressure on what Europe ought to be or what it should not be", he said.
The Prime Minister is also expected to say that the United Kingdom will continue to help with Europe's migrant crisis and will be a "reliable partner" after Brexit, says the BBC.
"We certainly learned from the history of the last years, that there will be as well a European Union with different speeds, that not all will participate every time in all steps of integration" Merkel told reporters after a summit in Malta.
It's not acceptable for the global community if America shirks responsibility [for taking in refugees].
"We need tax revenues, we need a fair tax system, in order to make necessary investments in our society".
A spokeswoman said: "She said that the alternative - division and confrontation - would only embolden those who would do us harm, wherever they may be".
The European commission's high representative, Federica Mogherini, said the EU did not "believe in walls and in bans", and claimed the union would be a reference point for the world.
"I already said that Europe has its destiny in its own hands".