Tusk even compared the US administration to Europe's traditional foes Moscow and Beijing as he launched his broadside hours before a dinner of the 28 leaders in Sofia where they will discuss the issue.
"Looking at the latest decisions of President Trump, someone could even think with friends like that who needs enemies," Tusk told reporters.
"But frankly speaking, Europe should be grateful to President Trump, because thanks to him we have got rid of all illusions. He has made us realise that if you need a helping hand you will find one at the end of your arm." The transatlantic rift has hijacked the agenda of a summit on Thursday at which the EU leaders will meet their Balkan counterparts in a bid to foster closer ties and keep Russia out of their backyard.
European ministers met with a top Iranian official in Brussels on Tuesday in a bid to save the Iranian nuclear accord after Trump decided to pull out. Meanwhile the EU is still trying to win exemptions from tariffs on steel and aluminium exports.
Tusk called for more unity in the divided EU -- which is set to lose Britain as a member next year -- to face the growing challenges.
"Besides the traditional political challenges such as the rise of China or the aggressive stance of Russia, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration," Tusk said.
"I have no doubt that in the new global game, Europe will either be one of the major players, or a pawn. This is the only real alternative." The 28 leaders will also discuss the deaths of dozens of Palestinians in Gaza after Trump moved the US embassy to Jerusalem, in another move that the Europeans had strongly opposed.
Tusk said he wanted the European leaders to "reconfirm" that they will stay in the Iran deal, as long as Tehran respects it -- although Europe must now find a way to make up for the US sanctions on Iran that Trump will reactivate.
European Commission President Jean-Claude Juncker and EU foreign policy chief will outline to the leaders what measures the bloc could take to shield its now substantial economic interests in Iran, Tusk added.
Merkel however said that Europe had no choice but to stick with the ties that have bound it to Washington since World War II.
"Despite all the difficulties we have these days, the transatlantic relations are and will remain of outstanding importance," Merkel told the German parliament.
Over the dinner of Bulgarian salad, meatballs, buffalo steak and walnut biscuits with berries, May meanwhile was to tell her fellow leaders that Britain was "committed to ensuring the deal is upheld" so long as Iran honours its commitments, her spokeswoman said.
On Trump's decision to impose tariffs on steel and aluminium imports, a move the EU has warned could spark a trade war, Tusk urged the leaders to keep a unified front. "I will propose we stick to our guns," Tusk said.
"It is absurd to even think that the EU could be a threat to the US. We need to bring back reality in this discussion, which is not the case today," Behind their message of unity and firmness, some member states seem open to tolerating limited quotas from the United States on metals imports while others want a harder line, diplomats said.
The violence in Gaza will also be on the agenda as it was "linked to the bigger question of the consequences of the decisions of Donald Trump," an EU official said. 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 US embassy in Jerusalem, the conflict's bloodiest day in years.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)