On Trump's decision to impose tariffs on European and Chinese steel and aluminium, a move the EU has warned could spark a trade war, Tusk urged the leaders to keep a unified front.
Listing Europe's traditional challenges, ranging from the expanding power of China to the belligerence of Russia, Tusk said: "We are witnessing today a new phenomenon, the capricious assertiveness of the American administration".
The 28 leaders will also discuss the deaths of dozens of Palestinians in Gaza after Trump moved the USA embassy to Jerusalem, in another move that the Europeans had strongly opposed.
"Looking at the latest decisions of President Trump some could even think, 'With friends like that, who needs enemies?'" "But frankly, European Union should be grateful", Tusk tweeted. "He has made us realize that if you need a helping hand, you will find one at the end of your arm", Tusk said.
The European Commission is refusing to open talks on wider trade terms with the U.S., including import tariffs on cars - a bugbear of the United States president - until it receives a permanent exemption from punitive taxes on European steel and aluminium. "But at the same time we must be prepared for those scenarios, where we will have to act on our own", said Tusk, a former Polish prime minister. This is the only real alternative.
White House National Security Adviser John Bolton indicated on Sunday that the USA would be willing to impose sanctions on any European companies that continue to work or do business with Iran.
The EU has called for "utmost restraint" after Israeli forces killed 60 Palestinians during clashes and protests along the Gaza border against the opening of the United States embassy in Jerusalem, the conflict's bloodiest day in years.
It was crucial, he said, that the European Union stuck to its guns and refused to talk trade with the USA until it received a permanent exemption from punitive tariffs on steel and aluminium imposed by Washington on the grounds of national security.
European Commissioner Dimitris Avramopoulos said the EU was willing to use legislation that would make it illegal for European companies to comply with U.S. sanctions imposed on Iran. Trump says the tariffs are needed for national security reasons.
"We knew that there was a great risk, that this decision to move moving the embassy would bring less security, bring tragedies, and sadly we were right", he said.
European leaders will gather in Sofia to mainly discuss the EU's commitment towards enlargement talks focused around Western Balkan regions, but the US President's actions have seemingly hijacked the agenda.