Speaking to reporters in Sofia, Mr Tusk said the US President had "made us realise that if you need a helping hand, you all find one at the end of your arm".
Mr. Tusk called on the European leaders of the countries that signed the agreement - like Prime Minister Theresa May of Britain, Ms. Merkel and President Emmanuel Macron of France, who tried but failed to talk Mr. Trump into not scrapping the deal - to reconfirm their commitment to the accord.
Speaking on Wednesday, Tusk said he was "grateful" for Trump because he had made the European Union realise "if you need a helping hand, you will find one at the end of your arm". The contrast has stirred up even more European anger toward Trump's foreign policy, and the Gaza violence is another mark on the agenda for European leaders to discuss during their day and a half of meetings that starts Wednesday.
"Following last week's announcement of the United States withdrawal from the Joint Comprehensive Plan of Action with Iran, we need a united European front", said Tusk who arrived in Sofia to participate in the EU-Western Balkans Summit.
And he said that the EU would work to develop ways to shield European companies that continue to do business with Iran from any sanctions or other penalties imposed by Washington. "Despite the USA hesitation", he said.
Mr Tusk said he wanted the 28 leaders over the next 24 hours to reconfirm that the European Union would stick to the deal as long as Iran did.
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Tusk's public comments on Wednesday also touched on the new USA tariffs on steel and aluminum that are set to bite on June 1 unless Trump grants a reprieve. European ministers met with a top Iranian official in Brussels on Tuesday in a bid to save the Iranian nuclear accord after Trump chose to pull out. The EU and U.S. are friends and partners, therefore USA tariffs can not be justified on the basis of national security'.
While European officials have spoken out against Trump's trade actions before, the language used by Tusk marks a new tone in the relationship.
The European Union must cope with the "capricious assertiveness" of President Donald Trump's administration just as it has to deal with the rise of China and Russian aggression, EU President Donald Tusk said.
Theresa May, Emmanuel Macron and Angela Merkel have resolved to stick with the joint comprehensive plan of action with Iran, despite the threats of USA sanctions against European businesses.
European Commissioner Dimitris Avramopoulos said the EU was willing to use legislation that would make it illegal for European companies to comply with USA sanctions imposed on Iran. In order to be the subject and not the object of global politics, Europe must be united economically, politically and also militarily like never before. "We need to bring reality back into this discussion".